Actually, I believe all the parts are indeed rated for -40 to +85 degrees centigrade on Teensy LC & 3.2. Before updating the website, I'd really feel better if I reviewed them all carefully.
But I did look at this a few years ago and the crystal was the only part holding us back from the wider range. The crystal we used on Teensy 3.0 & 3.1, and still use on Teensy 2.0 has a narrower temperature range.
Of course, Teensy does have some small self heating due to internal power dissipation. This varies depending on the clock speed, and which peripheral are enabled, and by whether the I/O pins are toggling rapidly and driving longer lines or other capacitive or resistive loads. As with all electronics, the self heating varies considerably depending on available air flow. The maximum usable temperature will be somewhat less than 85°C, depending mostly on the surrounding environment.