Foss navigation with traffic?

There's nothing that stops a FOSS app from doing that

I'm well aware there's no technological obstacle. many people choose FOSS maps over Google to have less of their data collected though, so I could imagine they'd object to such practice, which would make the feature less effective.

You'd have to trust whatever host, but if the actual server was handled by the right reputable organization, anonymizing information should be relatively easy (if and probably only if you had a sufficiently sized user base). You basically would just have chunk up the data when you send it, fuzz time on the order of a minute or five, and make sure it didn't have any ID attached to it in any public data sets.

With a smaller user base, you could maybe make deanonymization significantly less valuable/harmful by allowing users to set dead zones locally that they don't share data from. So maybe in a city you set 2 miles from your apartment and in a more urban area you set 5, and the same for your place of work or other areas that you consider potentially invasive to a bad actor. A small base would likely still allow your route to be correlated, but removing enough of the ends cuts the risk down a lot in theory.

It's something you'd have to stay cognizant of, but it might be able to be done in a privacy protecting way.