Hi Basile,
I think that the error you get comes from a transition from opam1 (used in the Patoline webpage for instructions, I guess) to opam2, which changed the "opam switch" behavior to stop installing a new switch explicitly. The command recommended in the error message, "opam switch create 4.05.0", will do the equivalent of the README's command, and get you a 4.05.0 switch. You should be able to build Patoline there, assuming the dependency bounds have not bitrotted away.
Getting Patoline to work on a more recent OCaml version may be very easy (just installing it in a newer switch) or very hard, depending on the change-robustness and maintenance level of its codebase and various dependencies. You cannot tell without trying.
Looking at the "issues" list suggests that Patoline is intermittently maintained by Rodolphe Lepigre, so I think you should get help if you open an issue on the repository to report on any further trouble installing Patoline, using either 4.05 or a more recent version.