Hey!
The text isn't rendering because I accidentally set the mask mode to be "Inside" by default. Set it to "outside" and it should render fine! I'll change this in the next patch, sorry about that.
That's odd that the SDF mode box can't be checked... as long as it's visible there, I can't think of what wouldn't make it toggle-able. I'm able to toggle it in the exact same shader, maybe try it after solving the above issue? It should make some additional settings appear in the inspector immediately. Let me know if that leads to anything?
Finally, the legacy SDF shader wasn't meant for Unity UI text, only normal text. Using any legacy normal text shaders on unity UI results in it rending all-black.
-Kai