The following file is meant to set a swap limit on init.scope:

# /etc/systemd/system/init.scope.d/override.conf
[Scope]
MemorySwapMax=0

This works on RHEL8, but has no effect on RHEL9.

While I could not pinpoint the reason for the regression, I have confirmed that the following systemd patch is missing from RHEL9 and does fix the issue:

b26b3dc6 (core: ensure init.scope is realized after drop-ins have been loaded)

It would be great if this patch could be backported to RHEL9 to fix this regression.