My main regrets are code for canceled closed source projects that will never see the light of day (waste of effort), and one early-career large scale refactoring which was only ever half-finished due to time pressure - resulting in worse code than if I'd never initiated the refactoring in the first place.
Even the bad technical decisions tended to be correctable learning experiences.
Yeah, this weighs on me too sometimes. I've spent man-decades of work time writing proprietary code for companies that will never see life outside of those companies. And many of the products I've written the code for have since been shut down, or never shipped in the first place.
I've recently gotten re-involved with an open source project that I used to work on in the mid-00s, and it's really satisfying to see code I wrote so long ago, still in use, and get to work on it again.
Even the bad technical decisions tended to be correctable learning experiences.