My expectation is that PS4 "native" saves will work fine on the PS5 in BC mode as theoretically they share title IDs. What I find frustrating is that if the game is an upgraded PS5 native client for the same game the saves will not work as with Spiderman MM PS4 -> PS5 and now Dirt 5.
If their save management API is not setup to easily automatically present PS4 saves to PS5 titles at an O/S level they should at least modify their API to allow new titles to implement PS4 save import processes. To my understanding of how PS handles saves the title itself is fundamentally not involved at all in cloud versus local, while I can from the Xbox interface specify I want to load from the cloud or locally (been a while since I played on Xbox please correct me if wrong/changed) on PS the game is completely cloud unaware and can only load files from the local store. Cloud saves is really more a cron job that simply replicates files on the disk into the cloud on a schedule so I wouldn't be surprised if it's impossible right now for a title to specify or request access to a given file store in the cloud at all. It seems simple enough to add a notional command "AccessSaveStore(TitleID)" that would allow companies to display saves from other stores so I find myself struggling to come with good reasons why they seemingly haven't done so, might this functionality be deemed a security risk if vm the title runs in is accessing storage areas that don't "belong" to it? Very strange to me given that for Sony even if they foist the work onto devs they can say "hey we gave them a way to enable you to continue to use your old saves".