As of this article's writing, HA Meowstic was unavailable. However, there have been trades on numerous forums for HA Meowstic, so it must have been released after that article was written.
Meowstic was originally impossible to get with HA in Sword and Shield, but then it was released with HA.
The Crown Tundra introduced an item called the Ability Patch, which gives a Pokemon its Hidden Ability. Thus, Meowstic can get its HA by applying an Ability Patch to it as well.
Sources: Experience, GameFAQs trades, Reddit trades, the aforementioned article.
Hope I helped!