Check whether the given user id localpart is valid and fully conforming
Err for invalid user ID localparts,
Ok(false) for historical user ID localparts
Ok(true) for fully conforming user ID localparts.
compat feature enabled, this will also return
Ok(false) for invalid user ID
localparts. User IDs that don’t even meet the historical user ID restrictions exist in the wild
due to Synapse allowing them over federation. This will likely be fixed in an upcoming room
version; see MSC2828.