Just tried.
If you have anything that uses components in your models, you will not be able to use PlanetScale for your DB because they don’t support FOREIGN KEY constraint (and will not). But strapi generates sql schema that uses foreign key constraints with ON DELETE CASCADE.
So the error will be:
error Error: ER_QUERY_INTERRUPTED: foreign key constraints are not allowed, see Vitess | A database clustering system for horizontal scaling of MySQL
So yeah, PlanetScale sounds like a good managed DB option but no it doesn’t work with Strapi. Unless, of course, Strapi drop the use of foreign key constraints.