XUL extensions e.g. the user interface could not be ported, the notice they were given was "this will stop your extension working" with users being given the advice from advocates "don't worry we'll build some replacement APIs sometime soon..."

There were years of notice, but no infrastructure to support addons migrating to WebExtensions. They're still working on the supporting APIs now, after throwing the switch. What would have made more sense would be to bring in support for WebExtensions and all APIs necessary to replicate popular addons first, give them enough time to migrate, and then disable legacy extensions.

Technically yes, there is the ESR version available until April, but the vast majority of regular users aren't going to know about that.