I have also seen and contributed to serious efforts to introduce various boot-time flags / variables that made the app look like something very different depending on that input.

Not super idiomatic, many would say, but it worked well, we just had to have a good discipline. Lint by lint, regex rule by regex rule, we got there. It worked great.

I don’t deny that umbrellas can do the same but they gave me enough WTFs back in the day for me to remember not wanting to risk with them again. Maybe it’s a skill issue but I’d be okay with that as well. I don’t aim to know 100% of everything in Elixir.

And yeah Boundary is great. I love it and used it many times now (ironically never in my OSS work but so far I have not needed it there).