I use a single flat text file, and use it for years. I have an editor macro that adds a new, timestamped entry at the end of the file, which looks something like this:
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27-Oct-2022 9:38:48 am
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The bar is where my cursor lands, all ready to start typing. A simple alias ("note") brings up the file. I can make a note in a second or two, then get back to what I was doing.I dump everything into this file. At my last job, my notes file was maybe 20-30MB and nearly a decade old. I record hard-won knowledge, TODO items, pieces of debug sessions that are interesting, little notes about what's going on, jokes and random thoughts, contact info, whatever. Editors load big files fast, and incremental search works great. The idea is to make this practice fast, nearly frictionless, and not adopt complications that are tough to maintain over time.
I've found that the less formal and structured I make things, the more notes I take; if I had to conform to a schema it would be a burden. No Emacs modes. No email or contact or TODO-system integration. Just notes.
It's saved my butt more times that I can remember. It's great to have at review time, too.