As an additional data point: Mine tunnels also (often) only have one entrance, are not meant for transportation from one end/exit to the other, and are also tagged with `tunnel=*`.
https://www.openstreetmap.org/way/227946737

Note that 'tunnel=*` does not necessarily mean it's for humans in the first place. (e.g. `tunnel=culvert`)

> make tunnel=* compulsory on all linear underground objects

If they are artificially 'housed'/'eingefasst' then yes. Not if it's a natural cave.