So I discovered and this is why I wrote this post  . Oh well...

Robert, that's what I initially thought too and it's indeed mostly so but not 100%: the old style %RI is actually calling the new style ROUTINE^%R, with Audit classes and all. Just no source control class suspenders  . I see no technical reason not to have them in ^%R but they are not there. The ZSAVE implementation is not available.
So this is why I am trying to tackle this problem from the Audit side. Any ideas? I am thinking of a process monitoring the audit trail records but I don't quite like this solution. In any event, is there anything beyond monitoring?

I don't want to close access but to run custom code when the same routines are modified via a terminal. Cache, and looks like Iris, do not trigger source control hooks when terminal is used, say, for ^%RI. Yes, I can close terminal access via the same Roles, but I'd rather not to do that if other solutions exist.

I already have custom source control hooks related to this issue but unless IRIS to which you link changed something in this respect source control hooks are bypassed when terminal is used for uploading routines. The cursory glance at the link hints that nothing has changed. 

After it got corrupt it stands at the first line for the duration of debugging. I doubt this was the design idea 

That's exactly the problem: the new breakpoint is reached but no yellow box and the cursor stays at the routine's first line. The code is executed though because the variable set right before this single breakpoint gets defined correctly as per Watch. I did follow your instructions except I still used the corrupt routine.
I doubt I can reproduce this corruption intentionally because that requires understanding why it got corrupted in the first place and that is still a mystery to me.

I'll remember your advice for mac or cls but my problem is with int routines. If I press View Other Code for them I see "There is no other code to view" and the yellow box still does not appear.

I did not find anything obvious how to do that. Could you explain it in more details or post a documentation link?

That means setting up, testing, and learning VScode plugin too which takes considerable time, I tried it before.

Robert, a bit more info after I tested a bit more:

1) The first program above, TestAG exhibited the same corrupt debugging behavior when I set it up as debug target for another project however this another project started debugging alright when I switched to a yet another debug target:

2) Test in this comment above leads me to think that the corrupt debug behavior might be associated with a program more than with a project. I don't think it is associated with the old style b break since it behaves correctly, the same as before.

I am not sure I am falling into the .obj code with this problem but maybe I do.  Just to be clear what is happening and what is not, a couple of screenshots.
1) TestAG^TestAG is the debug target, hand-written routine, and I pressed Go once, so presumably the debugger now sits at this breakpoint. However, no yellow box and the cursor is one line up. Shift+F11 does not make the cursor or the yellow box to appear anywhere else (I have other breakpoints too).

2) This is normal behavior. last is the debug target and the cursor correctly stops at the programmatic breakpoint with a suitable yellow box.

So any advice how to fix it?

I was debugging INT code successfully when it suddenly stopped working as described in the OP. Since then it is reproducible for this specific project and recompiling does not help.

Compilation started on 01/19/2021 15:05:17 with qualifiers 'cuk /checkuptodate=expandedonly'

It was non-generated INT code in the first place. I did try VSCode before but so far prefer Cache Studio.

That was, of course, it. I was so used to our "in-house" SQL with double quotes around strings that it never occurred to me that the outside world uses single quotes. Thanks, David!

@Alexey Maslov The ultimate goal is to create identical globals on 2 different machines with a single run, on demand.  I can run code on the first machine, then export global creating code which changes regularly, reconnect the Studio to a different machine, import code, then run it again. This is decidedly not a single run. I can also export and import global too, of course. I am willing to learn functionality that would be faster to deploy than these two approaches. Ideally as fast as ZN which unfortunately works on the same machine only but I might be wrong about that.

Marc, I'll consider going the Web Service way, but I am not a total but still a beginner in Cache. I never used Web Services before so I would need to learn that. I wish there would something as straightforward as the RemoteResultSet queries which take just a few short and obvious lines of code, no learning curve.

Mike, @Alexey Maslov  is exactly right: no licensing for ECP either, just checked.

More importantly, as I just found out, "mirroring is unavailable for the current license"

@Katherine Reid My goal is to update two globals on two different machines at the same time. That needs to be done from time to time only. Would or could "other async mirrors purposes" include that? If so is there any technical reading or specific classes, or the namespace setup, related to async mirroring that you would recommend to use for such a job? For example, this 101 guide, https://community.intersystems.com/post/cach%C3%A9-mirroring-101-brief-… , specifically states "Deploying a Caché mirror does, however, require significant planning, and involves a number of different procedures. And like any other critical infrastructure component, the operating mirror needs ongoing monitoring and maintenance." Given that I simply need to keep two globals in sync the Export/Import process seems much faster that what is described by this mirroring quote. 

@Evgeny Shvarov If by mirroring you mean this, https://www.intersystems.com/resources/detail/intersystems-cach-databas…, it seems like a huge overkill just to keep two globals in sync across network. This is not a backup situation. If mirroring could be scaled down to such a task, I am all ears.

I looked up your single class under your third link and it's probably a total newbie question but I don't see where would you specify login info for the remote server.

I'll look up applications under the first and the third link, thanks. The middle one is specific to IRIS, just checked, and I have Cache. I was more hoping for a short set of commands but if that is not available, oh well.

This is indeed the place for current local variables, thanks! However it has a minor and a major problems in my Cache 2017 instance. The minor problem is that I need to right click in the area to display those variables which disappear again with the next debugging step. So it is sort of semi-automatic. The major problem that this action freezes Studio pretty quickly. Maybe this freezing is fixed in subsequent releases, I don't know.

That's exactly what I need, under Local Variables!

Watching variables by typing them as well as hovering works in Cache Studio too. I am interested in automatically appearing (and disappearing) watch variables. Basically the debugger flow in this respect would be:

s a=1 ; When debugger is on this line show value of a automatically; don't show c yet; showing value of b is optional
s b=2 ; When debugger is on this line show value of b automatically; showing  value of a is optional
s c=3 ; When debugger is on this line show value of c automatically; showing  value of b is optional; stop showing a

Now, I don't know the exact Visual Studio rules when they start and stop showing a variable but the current line variables are always shown. No hovering or typing needed.

Yes, yes. all that is good and I use it but that comfort, arghhh...

Schönen Abend aus Boston,
Anna