Today Arduino released IDE version 2.0.0, with autocomplete, faster compile, and many other improvements.
Teensy is supported using Arduino’s Boards Manager. A separate installer is no longer needed.
To install Teensy on Arduino IDE 2.0.0, click File > Preferences. In “Additional boards manager URLs”, copy this link:
You can copy this link directly, or click the icon to the right to expand the “Additional boards manager URLs” to an easier to edit window.
Then click the Boards Manager icon on the left side.
In the boards manager column, type “teensy” in the “Filter your search…” box. When Teensy appears, hover your mouse to expand and then click the INSTALL button.
To start using Teensy, either select your board from the Tools > Boards menu, similar to Arduino 1.8.x, or if a Teensy board is connected to your computer, select it in the drop-down menu of detected hardware.