Because of AI bots ignoring robots.txt (especially when you don't explicitly mention their user-agent and rather use a * wildcard) more and more people are implementing exactly that and I wouldn't be surprised if that is what triggered the need to implement robots.txt support for FediDB.
It is not possible to detect bots. Attempting to do so will invariably lead to false positives denying access to your content to what is usually the most at-risk & marginalized folks
Just implement a cache and forget about it. If read only content is causing you too much load, you're doing something terribly wrong.
While I agree with you, the quantity of robots has greatly increased of late. While still not as numerous as users, they are hitting every link and wrecking your caches by not focusing on hotspots like humans do.