There are at least two different commercial hosting sites that implement a pull request model. If you want ephemeral branches like in git, hg branches are indeed not the right choice. But that doesn't mean that they don't have their place. Try topic or bookmarks if you want gitish behavior.I have absolutely no clue what you mean with trunk-based development...
Funny, I rarely use branches (or topics) in Mercurial unless it is a complicated long-term project. Modern "everything on a branch" was invented by git users because the UI forces naming things. Mercurial allows sharing code and still linearizing history with rebase safely, so much less need for merges. I've always found pull requests to be only useful for the passing-by contribution. They are a pretty awful interface for anything else driven by GitHub internals more than anything else...
> Pull Requests were never going to happen with Mercurial
TBF, AIUI they're not, strictly speaking, happening with git either: They're an external addition, invented by GitHub or some such, and not actually part of git itself.
How does that work; is it something technical built into it? Or do you mean just because after that one knows that it's incorporated upstream, so not needed as a separate entity any more? Because that would also go for a "pull request" by, say, e-mail or whatever.
It's much better than Subversion that was the standard before git arrived.
And it's much better than Mercurial that was competing to become the source control system to replace SVN.