This might not be what you had in mind, but the web framework Django. It's been around for ages and doesn't change in dramatic ways. It's not perfect by any means, but it just works and is reasonably easy to understand and extend if you spend the time to read through its code.

The documentation is good, but I find that it's a little overrated. There are lots of gaps, so spending the time reading through Django's code will pay dividends, especially when it comes to customizing it for your needs.

After years of churning through all sorts of Javascript frameworks, I know that I can build something in Django and only need to spend real maintenance time once every 2 years or so during an LTS release.