Make a track segment behave like a lightweight zone

This is a good idea, I've used this feature in other tools. One feature that isn't quite this but might be easier to implement and help address the same need is to make it fast to toggle between the assigned minimum, nominal, and maximum trace widths while routing.

That would also help, yes. Zone-like filling would be nice because it would minimize the narrower part length while maximizing its width, without need to think or define how wide exactly it can be.

Maybe it could also allow some kind of zone replacement: draw a wide track with highlight collisions/allow DRC violations. Select the track, change the minimum width.

Certainly this wouldn't be easy to implement. Interacting with DRC rules and interactive routing would be difficult.

Isn't the request similar to what other tools call automatic neckdown? (Which i would have expected to be already requested but the limited search functionality of this bugtracker only got me to https://bugs.launchpad.net/kicad/+bug/1799968)

It is similar but not the same (at least to automatic neckdown in Altium, not sure about other tools). Automatic neckdown in Altium changes the segment widths when needed, but does not fill like a zone. Filling like a zone would cause there to be more copper deposited when squeezing between round pads (like in a BGA) than just necking down the segment, although the difference is minor. I think automatic neckdown is also a useful feature, probably provides 90% the benefit of "zone traces", and is easier to implement technically. It might be good to add that as a separate feature request.

The use case in my mind was squeezing through a narrow space between pads and continuing the trace after that. As far as I understand, "neckdown" means track/pad connection. Zone-like track could take care of both situations. But on the other hand neckdown should typically be applied to an area (several track/pad connections at the same time), not to one segment. If neckdown could somehow be applied to segments passing between pads it would be enough for me.

KiCad bug tracker has moved to Gitlab. This report is now available here: https://gitlab.com/kicad/code/kicad/-/issues/2400