This also doesn't only apply to this particular changeset, but all of them. There is no problem mapping route relations if they have some sort of identifiable remains. The problem is all the sections that don't exist on the ground that are mapped as real features, such as https://www.openstreetmap.org/way/935480741 that just goes across fields, highways, properties, forests. These ways should not be mapped as railway=*. OSM only maps what is on the ground and these don't exist in any identifiable form in most of the places. An untagged way with relation would be fine.

And routes like this are not directly bikable, so I don't understand the argument here. A handicapped person could certainly never use any of these. Even the "best" example as mentioned below has sections where you have to take detours difficult even when biking, let alone if mobility is impaired.

And as rich says, there are plenty of cycle routes along former railway infrastructure where it is actually possible and restored/repaved ones are called Greenways, for example https://www.openstreetmap.org/relation/11003258. And yes, you can map something like historic https://www.openstreetmap.org/relation/18002788 here. And there are examples like this where it would match closely. But these are the minority.