Up until a few months ago I used Workflowy, for years and years. It's honestly a pretty darn good tool, mostly outlining but with all kinds of other features (linking, tagging, etc).

But recently I transitioned to just plain text files. I use Notepad++, because I'm familiar with it, and it's easy to incorporate folding features (I use square bracket to indicate foldable chunks, you can use what you want). This gives me something similar to outlines.

So far I'm liking it, a lot. I may set up some kind of personal server, so I can access the files on my phone, but it's not necessary now.

I'm trying to get away from ALL online services, so that even if my laptop doesn't have connectivity, it's still a useful tool to me.