Conpherence monograms are very rarely used (only email, I think) and threads don't have properties which are conducive to making monograms useful (i.e., it's currently very rare to reference a thread like you would a task or revision).
"E" is a better fit for events, and events have monogram-friendly properties, so freeing it up makes a lot of sense.
There are a couple of other free letters (A, J, O, X) but "Z" seems as good as they are and unlikely to be a better fit for anything else we're planning to build.
Possibly worth considering is using a symbol and letting threads have channel-like aliases so you can get a link to join the `%engineering` discussion or whatever, but we're pretty low on usable symbols and I don't anticipate this coming up a whole ton. You can always just link to the thread itself using a normal URL, and I think thread links will be most common in places where that's fine (e.g., wiki pages).