Back in platform update 16 in March 2018, we introduced a new metadata validation based on observed issues. We had found some customers (and also Microsoft code) had field groups that referred to fields that didn't (no longer?) exist - but our metadata validation did not check for this. Especially when upgrading code from older versions, those situations are easy to get into. Since we realized there were a not-insignificant number of these issues in the field (and a few in our code), we decided to introduce this check for bad metadata as a warning.

So hopefully, everyone is looking at their compile warnings in their code.

With PU35, we are now turning this into an error. As usual, this does not impact your running environments. It's important to re-iterate that you can update UAT, Production, etc. to PU35 without issues, even if you have some metadata issues. However, after installing PU35 on your dev or build VM, you will see compile errors for any field groups referring to non-existing fields and they will have to be fixed to be able to compile and package your code.