Guessing the upgrade did iris start [instance] nostu to not run any user code on startup.
iris help start
                            **** iris usage ****
Syntax:
        iris start <instance name> [parameters]
Description:
        Run the instance's irisstart procedure to bring InterSystems IRIS up.
Optional parameters:
        quietly -> non-interactive, with minimal dialog
        nostu   -> don't run startup routine (^STU)
        help    -> list supported parameters
        EmergencyId=username,password -> start up in emergency mode
        Any other value is taken as a configuration file name with '.cpf' appended.
        If a configuration file is not specified, the file 'iris.cpf' is used.
Example:
        iris start mystuff quietly      <- uses file 'mystuff.cpf'
Notes:
        Some instances might not have support for a particular parameter,
        or a new parameter could have been added.  For a list of supported
        parameters for a specific instance, type:
                iris start <instance name> help

There are some clear warningssugestions in the documentation but very reasonable.
Customizing Start and Stop Behavior with ^%ZSTART and ^%ZSTOP Routines - Design Considerations
Using the ^ZMIRROR Routine
One issue to consider is on Failover or DR mirrors the code might need to be in ZMIRROR not ZSTART.

I don't think you need more than normal production code awareness.

I would expect the WIJ to close to force it to the disk about every 80 seconds ie every write cycle.

my quick testing:

$while true; do
> ls -l IRIS.WIJ | cut -f 10-13 -d ' '
> sleep 40
> done
Oct 26 12:26 IRIS.WIJ
Oct 26 12:27 IRIS.WIJ
Oct 26 12:27 IRIS.WIJ
Oct 26 12:28 IRIS.WIJ
Oct 26 12:28 IRIS.WIJ
Oct 26 12:30 IRIS.WIJ
Oct 26 12:30 IRIS.WIJ
Oct 26 12:31 IRIS.WIJ
Oct 26 12:31 IRIS.WIJ
Oct 26 12:32 IRIS.WIJ
Oct 26 12:32 IRIS.WIJ
Oct 26 12:34 IRIS.WIJ
Oct 26 12:34 IRIS.WIJ
Oct 26 12:35 IRIS.WIJ
Oct 26 12:35 IRIS.WIJ
Oct 26 12:36 IRIS.WIJ