If you migrated to PS5, they informed us this is a one-way transfer, and you can't go back, as I understood it, to your PS4 copy of the game. This was similar to the upgrade to PS4 from PS3 where players lost access on the old console after migrating.

If you haven't migrated, then we know at this point that PS4 support is currently not being dropped, nor would I suspect it will be while the shortage on hardware is ongoing. I would, however, expect that this will be addressed and support will be dropped with a 7.0 expansion.