Comment # 5 on bug 1228833 from Thomas Blume

(In reply to Michael Hirmke from comment #4)
> Thx - I read that section, but "undesired behavior" is not really concrete.
> What may happen?

You can find a good explanation here:

https://github.com/systemd/systemd/issues/33083#issuecomment-2191889790

You might not see a difference to older systems since user session freezing was
implemented with systemd version 2.5.2.
With:

SYSTEMD_SLEEP_FREEZE_USER_SESSIONS=false

you just restore the old behaviour (before systemd version 2.5.2).
So, if you had an older system and suspend was working there it should also
work with the above setting.