Note that this will probably make the anon options differ from the default options again, in order not to change the parser cache keys. So (at least initially) ::getDefaults() won't change, but ParserOptions::initialiseFromUser will call ::setUseParsoid(true).

We probably need to have a migration strategy to avoid breaking CI/tests when this change happens. Probably by adding explicit ::setUseParsoid(true) calls to tests which depend on Parsoid output.

This will need to be gated with $wgUseLegacyParserByDefault as per T419048 but tests should override this to false.