That said there are languages where very little backward incompatible changes are made. Java comes to mind.
Also: putting each client in a sandbox (VM, VPS, etc) can help you to keep some clients on the old versions, and/or do PHP interpreter migrations on your own timeline.
I do and appreciate the job security but I respect my clients (small biz owners) and it's just not satisfying and for the 10% that have issues, costly.
Java used to be like that, but the migration to the module system has been very rocky for me personally when revisiting older projects.