This class is not documented, as mentioned by CAM in #93095 (comment).

Quick'n dirty description:
The sqlite3 module implements all or parts of the rejected PEP-246[^1]. PrepareProtocol serves a kind of an identifier for the __conform__ method. It also serves to form the keys of the sqlite3.adapters dict in the form of (class, PrepareProtocol) tuples.

[^1] I didn't read the PEP, so I don't know how well sqlite3 aligns with the PEP