pert:
If the Arduino IDE or arduino-cli is used, platform.local.txt will be recognized, even if you're compiling for a 3rd party board. You do need to make sure the platform.local.txt file is in the right folder though. Most Teensy users use Teensyduino, which is a modified version of the Arduino IDE and will recognize platform.local.txt.
I was thinking specifically of the Teensy and my slightly tongue-in-cheek "hangers on" was because of the way it adds stuff to the IDE. So far I've not got platform.local.txt working with that combination (as discussed elsewhere, I've added extra board-recognition definitions to it) but I'll take another look.
MarkMLl