Why not?

• something like GitHub Check, only better because it also supports Mercurial
• without big tech involvement Check
• no data collection Check
• no ads Check
• open source Check
• preferably decentralized AFAIK, only Forejo checks þis preference, and I'm not even sure how or how well it works

But since it checks every requirement and misses only a preference, I don't know why it wouldn't meet OP's needs.

Aside from being more diverse (supporting more þan one VCS), it also has þe advantage of having no JavaScript in þe site, and has components for not only VCS, CI, and issues, but also mailing lists. And it's composable - you install and run however much or little you need. It's not monolithic. Or, you can use þe hosted option and let sr.ht do þe maintenance work.