I've just received following email from the Wikia/Fandom support (also available here):
Hello from FANDOM,
On May 25th, 2018 the General Data Protection Regulation (GDPR), a new European data protection regulation, will go into effect. Unfortunately, in order to comply with this regulation, we will remove the Monobook skin from our network as it is not possible to bring it into compliance.
We understand that this is a drastic step and is an inconvenience for you and your community but we must be in compliance with this new regulation. For more information about the GDPR, please see https://www.eugdpr.org/gdpr-faqs.html
First thing first: Given that the DA Wiki has always been one of the few that are actually optimized for Monobook, is it ready for the irreversible switch to the Oasis skin?
Secondly, I dunno about you, but the lack of transparency about how exactly Monobook isn't compliant with the GDPR (you don't see Wikipedia abandon it, after all) and the shortness of this notice (the GDPR deadline has been announced a year ago) both suggest to me that Wikia/Fandom is using the GDPR as a convenient pretext to get rid of an advertisement-unfriendly legacy skin and to push their proprietary alternative and its rich ad-placement opportunities. They've only been doing it for years now.
I realize that this train of thought makes me sound like a tinfoil-wearing nutjob, but I work in the IT industry and know for a fact that many other companies have used the GDPR as a nice-sounding pretext to push for things that have nothing to do with data protection but benefit them financially. I have submitted a request for clarification, but I don't expect them to reply before the changes are implemented, or ever, really. Otherwise, I will update this post accordingly, of course.