No changes over the behaviour of PRs any time soon, I’m afraid.
It’s always an option to set up two sites, one as a staging environment and one as production where only changes which are merged to the production branch get pushed to the production site. Then, you could run a cleanup on the staging env site ad-hoc, or periodically.
One concern is, are all PRs only reachable with the permalink (i.e. is there a robots.txt)? Wouldn’t want a PR deploy showing up in a search result for whatever reason down the road
Consider features like this! Very easy to avoid duping data with canonical headers.