Flixbus routes are not as fixed as city bus routes. In principle I think they have a coarse route, they obviously have fixed stops, but in between they can drive better routes according to cicumstances. The few times I took a Flixbus long distance, the drivers used regular navigation systems.
After an update, the lines are now visible, combined with local bus routes. There are currently 35 there. If nobody is maintaining them, I’d mark them as historic.
“Historic objects with no relevance to current state of an object generally do not belong in OpenStreetMap.”
(One exception are objects that no longer exist but are still visible on aerial images.)
Therefore, i suggest removing former Flixbus routes.
As for current Flixbus routes (or other international bus routes), i am in favour of removing the road members and only keeping the stops, for the reasons of unstable courses, minimal benefit and difficult maintenance.