Based on my understanding this would simply make it very difficult to run google play services (or anything using them) on a custom ROM.
This would make it incredibly inconvenient to run one. Since users would pretty much be limited to something like F-Droid for all of their apps.
But it won’t “end” custom ROM’s.
There are people using them. Despite no support or limited support for Google Play Services in many cases.
But it does change the landscape and not in a good way.