Allow me to pick your example apart a bit.
However, what is not feasible, or within the realm of reason, or what privacy advocates tend to advocate for, is the ability for me to willingly go up on stage, say something on the mic which I immediately regret, and then ask everyone present to forget it ever happened and delete any footage they might have of it. No reasonable person would ask for something like that, because it is not a reasonable request.
That's not what is demanded. No one demands that the audience (users) forget what I said (the comment), much less: immediately. No one is asking for mind-erasing power or the ability to remove screenshots from other people's client devices.
With federated websites, that becomes enhanced; it's like if the bar you're in has a camera pointed at the microphone, and transmits both video and audio directly into several other bars.
Now, that is where the actual demands come into play: As you pointed out, it is reasonable to demand that the bar deletes any recording of what I said on stage. But the way the footage is shared with the other bars can be regulated via a protocol. In your analogy, it's like the other bars copy tapes from the original bar and show them at their place. Now, implementing a procedure of "delete that tape, please" is not impossible. In fact, it already works on Mastodon. If a bar doesn't comply, it simply wont get any tapes from the other bars (it gets defederated).
AFAIK, there is already such a feature planned on github. Which is great. But that is exactly the reason why these things need to be brought up and "privacy realism" is counterproductive.
It could defederate any non-compliant instances.