I think it is renamed to distiguish the paid play store version from the free f-droid version which is technically the same app.
F-Droid
F-Droid is an installable catalogue of FOSS (Free and Open Source Software) applications for the Android platform. The client makes it easy to browse, install, and keep track of updates on your device.
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I found this commit with name changing.
But no further clarification, only:
Clarify osmand naming
Dated: 29.08.2012 at 12:32.
Ironic
So that's when they introduced wavy roads
know for sure but my guess is that the + version is the paid version on play store with unlocked features. Since this is the full free version on fdroid they changed the name to clarify the difference.
Edit: Found on Wikipedia under licensing that this does seem to be the reason. https://en.m.wikipedia.org/wiki/OsmAnd
To my understanding this is the correct answer
Osmand+ is the paid play store version, Osmand~ is the open source version. I believe there's no significant differences.
Osmand open source version does not support android auto
I've been wondering this for a long time too actually. Perhaps the name was taken?