Short version.

No you cannot login with facebook anymore.
Though "effective Janurary 2021" The accounts did not seem to have issue until recently.
Which fits the timeline and makes sense.

However the accounts are still there. Accessible. Just can't login.
Well ya can but..
Yeah I know your issue.


Start with that. They will be able to get you set up.
Account access
Account linking
The whole "migration" process can be done through them.
We have to log in to Uplay, it's their support.
That's ubisoft.
Which you may not know but is one of the giants in gaming.
Owns this rinky dink junk of rusting metal I know. It's a unique thing in gaming history.

It's like having the best NES game collection. Metriod, mario 1 2 3. Rad racer. Zelda 1 2. You've got em all! But then you have that one cartridge.
That one.. thing..
Action 52. AGH!
Oh well.

24/7 live chat support.
Email support.
And if you dig for it, you can find the phone number and get in that way.
They are paid to help and will help.
@castleempires email is no longer active.
Though internal support tickets are still active.
My guess is this is handled by BB, as they would be the ones handling the game itself. Ubi just owns the IP rights, and accounts.

So really. The issue is.
You don't go asking ubisoft for advice and help with game issues.
So you shouldn't be asking BB for help with your Uplay login. We have to ask Ubi.
Sorry this is not made very clear. But I think this will help.

I have in past contact Ubisoft and changed both my email address and password associated to my Snaketeeth01 game account.
So I know they can handled migrations.
If you have evidence of your accounts still existing.
Know all the emails and passwords.
You may just be golden. I was lucky enough to have a CD key associated with my Uplay which I used as 2 step authentication.
I cannot speak to what chat support will ask of you to provide. But I do hope they can help.

If the provided link does in fact help you to migrate your account. Please let me know below. Good luck team.