Backwards compatibility is a cornerstone of WordPress core development philosophy. It is, unfortunately, not something nearly enough plugin or theme developers take seriously. When a plugin or theme project gains 10s or 100s of thousands of users, backwards compatibility can be crucial to the overall health of the project.

Ensuring backwards compatibility is more of a mental mindset than anything. Developers have to mentally make the commitment and say to themselves “I will NOT break installs during upgrades”.