To my knowledge Sourcehut is strictly based on mailing lists and email patches, which is by no means obsolete (Linux still uses them for a good reason) but old-fashioned compared to say, Github PRs, limiting a pool of potential contributors.
Drew wrote an interesting piece on the mail-based workflow (I linked to it elsewhere on this page). Yes it's been around for awhile, but it also seems like it really works better for certain use-cases.
