Interesting. I don't see why Monobook would be less GDPR-compliant than Oasis', specially since Monobook has zero social tools (which means zero or almost-zero privacy concerns compared to Oasis').
FANDOOM has a lot of reasons to ditch monobook, being ads and its maintainability probably the most important ones. But GDPR? Come on
This, however, no longer affects me, since I don't edit here anymore :)