Here is some documentation on %G and other global utilities, as well as Routine utilities.
https://docs.intersystems.com/priordocexcerpts/prgglobalchui-41.pdf
https://docs.intersystems.com/priordocexcerpts/prgroutinechui-41.pdf
Yes, this is absolutely correct, the code checks the R/W status of the database. If it is set to read-only, it will not be copied up to the durable directory. The first version which has this feature are kits based off of IRIS 2021.2.
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It is available in 2022.1, it just hasn't hit the documentation yet. You can go ahead and use it.