Michel Anders writes:
When you create a lot of add-ons over time, you are likely to encounter a few that are absolutely ancient (you know, before Blender 2.8x :-). So where do you start when you want to upgrade them?
Blender and its Python API are changing all the time, so the differences between 2.78 and 2.93 will be huge, right? Yes and no: they are extensive, with lots of new functionality and quite a few breaking changes of course, but may be not as many as you would think.
To help out a little bit, I collected all those changes along with pointers to the relevant documentation in an overview article. It's probably not complete still, but it is a good start and the issues are sorted in the order that you are most likely to encounter them.
(warning, long read, but hopefully it helps)
[Article image created by me: mug, lamp, pencil&pen CC0 by blendernoob, Kilt2007, & lakerice respectively]