Sure it was a mistake by OP in TFA but it's an honest mistake any of us could make. And it's a really Bad Move by the Mongoid devs. It'd be a bad move in 1.2->1.3, but by 7.3? I'd just be like "whelp, this is how this function behaves now in perpetuity, I guess just document that it has different semantics than AR" even if an 8.0 was released. This is the kind of change which causes really subtle bugs. It's rarely worth it. Just introduce .chain_or() or something.
It's really just as simple as going to the release notes and seeing if there's a mention of breaking changes or deprecations. If no such thing is mentioned then you're fine, otherwise you go and see if that change affects you at all and just take a little bit more time to test the upgrade. This has been standard practice at plenty of places that I've worked.
It also doesn't mean that it makes the change a particularly good one, but I don't think I can make a bunch of OSS maintainers responsible for my own failure to review an upgrade and test it before rolling it out. In those terms, there is some culpability both with the maintainers and with the library users.
Integration tests might've done the job here.
You'd be surprised how far people go to shoot themselves in the foot.