Are there any plans on implementing XEP-0126 (invisibility)? Thanks!
XEP-0126 is just a set of guides showing how to use XEP-0016 Privacy Lists (which Sleek does support) to create the invisible effect, so you can start using that now if you need it. I'll add 126 to my list of plugins to focus on next.
There's also XEP-0186 (Invisible command) that Sleek supports, but that also depends on server support.