How should this paste have been matched? It doesn't seem to be used in T96927.

In general, I would suggest to use Maniphest search, but that doesn't seem to work here, as Pxxx is also used as wikibase identifier...

Actually, there seems to be a link when it's mentioned. See for instance P631 at the bottom. Does that solve your issue?

That's the thing, it needs to be mentioned, which I guess is the norm, but is a step which may be missing, as per my example. Does the interface for uploading one ask anything about tasks?

No, the paste is just a paste, like other pastebins around. I'm not sure how anyone would know about the paste though, if it isn't mentioned in the task.

Exactly like I found out about my example: direct link.

I'll mention my pastes in future tasks to make sure there's a mention :) Sorry for this.

I'm not sure a technical solution is needed; the mention system seems enough. Perhaps some friendly nudging would be more efficient if you find that other people overlook the mention step. It worked for me :)

You could (ab)use forking for that (see P587 which I forked to P641)

Linking to an existing pastebin from a comment is just adding a string starting with P, like this P256.

@gpaumier I wasn't making a case out of your paste - I wouldn't bring this up if this hadn't occurred to me before.
My suggestion was an optional "Associate task(s)" option while the Paste is being uploaded, to lower the risk of it being "orphan" (it would auto-mention the Paste in the task itself, so people might use it to avoid the extra step of having to update the task themselves).

fwiw I also think that the current setup is good, based on mentions and the fact that you can associate pastes with projects.

PS: since we talk about linking pastes, it is worth mentioning the very elegant {P587} format: