If the quoted text is deleted, then reply to that quote will automatically be off-topic. So try reporting it.
Mod can edit the quote (just as they leave feedback.)
Unfortunately, aprt from one extra quote, the OP of the thread also quoted it, not sure if it would be regard as offtopic with the context of the quote body.
You can already do this with some creativity:
How about
And that shows the problem: anyone can create any quote they want.
True, didn't even think about this.
Being deleted doesn't necessarily make it a bad post. What if it was deleted to merge posts? That doesn't make the quote bad, but a red warning makes it look that way.
Oops, that's another view. The more I think about it, the deeper it gets.
The main blocker now is what you've mentioned in the first quote, that the quotes can be edited. If not for that, then editing manually by a mod using this:
When the page is rendered subsequently, the quote remain normal but the initial harmful content is replaced by your message DELETED BY A MOD
....could have worked, but then persistence across existing quotes (the affected post) and maybe the introduction of new complications is also another thing to consider..
But I don't think it's helpful to verify the quote, because it's easy to manipulate the quote. Rather simply, you can archive the quote and post the link as well, besides quoting someone's post. So we can verify whether the quote is authentic or not. Your suggestion would just increase developers' jobs and not help forum users, though it could be simply implemented by a line of code about return page.
Yes, you're right, but the issue is that no one knows the reason it might have been deleted when it's being clicked and nothing shows up, as long as it's a single post and not a thread. Unless it could just redirect them to the page showing the reason why the post was deleted. So even if anyone already quoted it and they try to see that particular quote, it redirects to that page (I am not sure every user will like this either, apart from those who won't even bother to narrow down to the post).