The expression [[Category:+]] in queries never worked. If you want to restrict query results to the namespace Category, then you need to write [[:Category:+]]. Restricting to results that belong to some (arbitrary) category is not supported.

Also, I am surprised that [[Category:*]] was supported before SMW 1.0. You can use ?Category as a printout to get this result in current SMW versions with #ask syntax.

In general, the beta compatibility mode was introduced to simplify the transition from early beta releases of SMW. Just like MediaWiki, SMW does not aim at arbitrary backwards compatibility with old releases, especially if they were explicitly flagged as "beta". Since the last beta version of SMW is much older than one year now, we will actually be removing the beta compatibility flag in one of the next versions. Accordingly, we will not fix problems with the old syntax parsing anymore. Even now, many new features/fixes are not available in the old syntax, so it is advisable to update any affected pages.