The language syntax is one thing, but Objective C also requires a pretty substantial runtime which probably is not a good fit for Teensy. At any rate, it would have to be ported because as I understand, the existing runtimes all target a POSIX/BSD OS, not bare metal or Arduino. I wonder if it could even be squeezed into T3.1 if someone tried.
For the size of code that will fit on a Teensy, C++ is not bad at all.