Most WordPress plugin development follows the same arc. Somebody needs a booking form, a feed importer or an extra field at checkout, a developer writes it, it works, everyone moves on. Two years later the site is stuck on an old version of WordPress because nobody is confident that plugin will survive an update, and whoever wrote it has gone.

The cause is rarely that core moves too fast. WordPress is conservative about breaking things, and plenty of well written plugins from five years ago still run unchanged on WordPress 7.1. Plugins break because of decisions taken in the first week: functionality put in the theme, core files edited instead of hooked, data stored in whichever structure was nearest, and nobody ever testing against a release candidate.

What makes a custom WordPress plugin survive core updates? Four things. The code lives in a plugin rather than the theme. It extends WordPress through actions and filters instead of editing core files.…