What you suggest is explicitly not a purpose of robots.txt per RFC9309[1]:
"These rules are not a form of access authorization."
HTTP 429 and HTTP 403 are what servers are meant to return to clients to slow them down or tell them to stop doing something without having first gained authorisation.
I wouldn't want them to. The whole point of using agents to do stuff on the web for me, is for them to do the stuff on the web for me.
This is the reverse of "do not track" case. It'll not be effective because every service will set it to DISALLOW by default anyway, because it costs them nothing, and for most services, it actually is what they want anyway - most of businesses on the web are making money on wasting people's time, and for that, they need to force themselves on people; end-user automation defeats that, so they actively fight it (and complain a lot).