We’ve built an API for adding custom sidebar sections as well
There are some examples in the comments here: discourse/app/assets/javascripts/discourse/app/lib/plugin-api.js at 00ab94bf53b784478d0aa7744bf3bb2d5b527580 · discourse/discourse · GitHub
This is marked as experimental but it should be safe to use (we use it to add chat content to the sidebar when enabled, for example)