I find the automatic "table contents" sidebar-like widget, that is turned on by default on most pages, to be a nuisance for any page with less than 5 sections/headers, when not using an ultra-wide browser window. It already auto-disappears on mobile formfactors (below 1000px wide), but on other resolutions it eats space for... not much. That is a little bit annoying on a 12" laptop screen or on a split-screen browser window, or on a portrait-oriented monitor. If it was docked into the main sidebar, at least there would be a reason for me to leave that sidebar open (and it would warrant that sidebar having a larger default width so that text doesn't wrap so badly like it does now); right now I end up having two sidebars most of the time so I always need to turn off the main sidebar and/or turn off individual pages' ToC property.
For what it's worth, Wikipedia's new theme (2022 version of Vector, which is what you see if you're logged in as a user) has the ToC combined with the sidebar, it's always-on but you toggle between the two, as you can see in the attached video.
Food for thought...
I'm guessing this would be possible only after the latest bootstrap migration (?), so marking it as blocked, but otherwise I would have considered it a "consultation / gathering feedback" idea... and I don't know how hard or easy this would be, maybe it doesn't even relate to bootstrap.