One of the intentional breaking changes in v0.123.0 is the introduction and usage of a logical path.
There’s a detailed explanation of how this change affects relative paths here:
https://discourse.gohugo.io/t/did-relref-semantics-change/48306/8
I recommend reading more about logical paths here:
https://gohugo.io/methods/page/path/#finding-pages
Specifically, the difference between a file tree and a logical tree:
https://gohugo.io/methods/page/path/#logical-tree
I’ll have a look at your {{< ref path="/posts/" outputFormat="rss" >}} issue later today, which you’ve also posted here:
https://github.com/gohugoio/hugo/issues/12096#issuecomment-1956558338