CurseForge replaced legacy API (without any forewarning) with a blackboxed API that requires keys.

So that's why MultiMC removed CurseForge support (in commit 0a827ba70e6ef20187f8507a536d54a8441020dc). (The commiter couldn't even bother to include the full explanation in the merge commit. This is why I hate GitHub.)

So scummy of CurseForge. They're also trying to insert ads.^["As we supported Linux, we tried to technically look on how ad monetization would look like on Mac & Linux (something that we didn't yet have the bandwidth to do).", Old API deprecation, Linux and Standalone client FAQ]