Castlevania: Belmont's Curse: cross-buy and cross-save
I found no official feature that moves progress from Steam to PS5. The one transfer Konami documents is from the demo to the full game, on the same platform and account.
Does Castlevania: Belmont's Curse have cross-buy?
Unconfirmed
No cross-buy announcement found. Each store sells its own copy.
Does Castlevania: Belmont's Curse have cross-save?
Unconfirmed
No cross-save announcement found. Konami's only stated transfer is demo save to full game, and it needs the same platform and the same account.
Source
provided you play on the same platform with the same account.
Limits of this entry
Konami's own support article returned an error when checked, so the quote comes from press coverage of it. The absence of a cross-save announcement is a negative finding from search, not a statement from Konami.