“bicycle:practical” or “Something below mtb:scale=0” sounds like a cycling equivalent of the discussions about “foot_scale”. Just as cycling has mtb:scale, walking has “sac_scale” which provides various levels of difficulty for hiking in high mountains, but doesn’t help much for day-to-day walking.
Anyone considering proposing something like this for cycling might want to start with these discussions and work back to earlier threads where relevant. I think by the time you’ve read all that, SomeoneElse’s suggestion to focus on existing tags may seem more appealing…