Hi there! Playing golden deer route, almost at the end of the game (past chapter 20, lost count sorry, it's been a while since I played) and recruited every single damn character (except Gilbert)
Here my blocked supports. Just to confirm with you.
Locked post-time skip:
Byleth X Claude (B support locked)
Byleth X Seteth (C support locked)
Linhardt X Caspar (C support locked)
Annette X Ingrid (C support locked)
Ashe X Marianne (C support locked)
Petra X Caspar (C support locked)
I don't play with the knights of seiros (Alois, Catherine, Shamir... Etc) or the professors that much so I wouldn't know if they have any locked supports. But so far this is all I gathered. Hope it helps.