Multicast has some serious issues (eg see #12993 and #12914) which are JDK and OS dependent, and essentially unfixable. In #12999 we changed to default to using unicast for discovery and in #13019 we moved multicast to a plugin to give users a chance to migrate their code.

We are not seeing many downloads of the multicast plugin which indicates that almost everybody has switched to unicast. I think we should deprecate this plugin in 2.2 and remove it in 3.0.