OpenACS 5.10 breaks nothing important (whatever "breaks" means). OpenACS.org runs the newest version of OpenACS 5.10.
When coming from 5.8, it is recommend to upgrade (at least kernel) to 5.9.* before going towards 5.10. It is not necessary, that everything works perfectly on 5.9, but the server should come up after the restart and has to be able to run further upgrade scripts.
OpenACS 5.10 requires e.g. newer versions on support packages (see https://openacs.org/xowiki/openacs-todo).
