I still use ed every day. Mostly non-interactive use, i.e., "ed scripts". Have not found any "modern" editors that are as small, fast and robust, especially for editing large files.

There is a book about Ed by Michael W Lucas. I guess, in the beginning it was supposed to be a bit of a joke, but it became useful for connecting the dots and is read for an afternoon: https://mwl.io/nonfiction/tools#ed

I've recently been doing some hobby coding on a Commodore 128 and I'm getting the hang of editing basic programs without being able to edit inline very well.

I use the list command to dump out sections of the program until I find where I want to edit and then add / edit lines whilst I can still see the listing at the top of the screen.

DBAs use ed, sed and related Unix utilities for removing create, drop and bad events from multi-GB dump and binlogs.

Source: DBA.

It’s ok for quick fixes. But it’s a different era of editing tbh