Not that long ago, its why I primarily use BSD/MIT/etc licensed software.

In what context? I currently struggle imaging a situation.

There is no obligation to upstream anything after all, you just gotta provide the code if someone asks... And why would you care as a user?

And you weren't happy giving out the source code? Even if you weren't charging money for this I'm struggling to be sympathetic

I could see it for things like electronics projects where the firmware gets modified as a form of configuration.

You might want to give out or sell customized devices, adapted to the specific user or deployment. The end user typically only wants the binary, so distributing the source is at best a good-faith-waste-of-time and at worst an opportunity to cause support problems.

The example I'm thinking of is keyboards running QMK firmware-- it's literally customized by editing a bunch of C and recompiling.

If you do this for your own personal consumption you aren't violating the GPL, and if you do this for other people all you have to do is give out the source code when asked.

Yesterday.

Example?