There was a website comparing the major RDBMSs to the standard with examples. I can not find it now but I found this:
https://en.wikipedia.org/wiki/SQL_compliance
Edit: this was it https://troels.arvin.dk/db/rdbms/
It's not updated though.
Arguably, timestamp with timezone should store a timestamp plus a tz_database time zone. Storing offsets is easy and useful but not necessarily correct. Timestamp with Offset should be a different type.
Validating all of those rules would be a hairy mess. For example, what do you do with a timestamp in the future when changes are introduced (negative leap seconds, changes to DST, geopolitical changes affecting timezones).
That's only a tiny thing, though. I'm sure there is more. I don't use Microsoft SQL Server. Quite happy with PostgreSQL (and SQLite for tiny in file things).