I no longer test with Windows XP. As far as I know, everything probably still works fine on XP. Most stuff probably even works on Windows 2000, except Serial+Keyboard+Mouse+Joystick (which depends on IAD, which Microsoft added in XP-SP3 and was never added to 2k).
It's unlikely anything that currently works will break on XP in the future.
Later this year, I'm planning to add 3 more choices to the Tools > USB Type menu. Odds are good those will work on XP, but if they don't, I probably won't invest much effort for the sake of XP.
Later this year or perhaps early 2016, I'm also planning to add some more features to the bootloader and Teensy Loader. Odds are slim that new stuff will work on XP. Odds are good anyone still developing on XP will be able to continue using the upload process that works today, but I do not plan to do much testing with XP.
Whether Java and the Arduino IDE continue to support XP are anyone's guess. I typically drop support for older Arduino versions after about 1 year. Teensyduino 1.21 will probably be the last version to support Arduino 1.0.5, for example. If Oracle breaks Java on XP and the Arduino devs depend on a non-XP Java environment, I'm not going to keep supporting legacy software for more than about a year, and I'm certainly not going to pour lots of work into the Arduino IDE or Java just to support XP.
My guess is anyone still using XP is probably already feeling the gradual loss of support from nearly all software. Teensy tends to use only very standard features, so Teensy support will probably outlast most other software.
Long term, developing code for Teensy on XP will mean living without new capabilities that will only be available on Windows 7 or later (and of course modern Mac and Linux). Eventually it may not be possible to develop code for Teensy on Windows XP.
Teensy programmed on newer systems, with any of the USB Type options that exist today, will very likely keep working with old XP systems.