The warning has been “softened” (Soften the 5.0 cautionary README note by dra27 · Pull Request #13394 · ocaml/ocaml · GitHub). My (personal) intuition is that supporting 4.14.* remains important for now, because there are users of large codebases which are still in the process of evaluating OCaml 5.x, but that nothing reported is unsolvable (so far 
). I would hope that by this time next year the cautionary note is able to be removed entirely. Note that existing known issues are in OCaml’s issue tracker, in particular the Performance and gc labels.
As ever, if one has a system using OCaml 4.x which for whatever reason cannot be switched to OCaml 5.x (especially if this is owing to a performance degradation), please do file an issue or start a discussion here, even if the system in question is closed-source (it’s obviously somewhat harder to investigate those regressions, but it’s still immensely useful to know the problems exist).