User blog comment:DaNASCAT/Technical Update: April 10, 2017/@comment-5956954-20170411011040/@comment-126761-20170411135912

In certain situations, switching achievements on/off could cause problems for the servers. It's an older system that we don't currently have plans to overhaul, so removing the switch from WikiFeatures limits the potential impact.

That said, as mentioned above, admins can request enabling/disabling the feature via Special:Contact.