A couple things to try:

1. A lot of thumbstick drift is caused from getting debris or sticky residue in around the thumbstick. This is especially common if you let your kids touch your Steam Deck/controllers. Taking some plastic-safe electronic cleaner spray (like CRC QD electric cleaner) and spraying it around the thumbstick while the deck is turned off can fix this pretty easily, and the same cleaner can be used to fix other controllers (especially nintendo switch joycons) as well as sticking controller buttons.

2. The steam deck has adjustable dead zones. If your drift is very minor, you can increase the size of the dead zones to compensate.

3. It’s possible to recalibrate the thumbsticks, see this guide

4. Replacement thumbsticks are available on Amazon, many people like to upgrade to hall effect sticks made by Gullikit or other companies. Ifixit has guides for both the right and left thumbsticks. Important to note with this, there are 2 models of LCD thumbsticks and 1 model of OLED joystick, you need to buy the same type of thumbstick as what your Deck already has. The Ifixit guides explain how to check which LCD type you have. Actually replacing the thumbsticks involves popping the back off the deck (with the microsd removed), disconnecting the battery, unplugging a ribbon cable from the thumbsticks, and then removing a couple screws before pulling the whole thumbstick out. After the new thumbsticks are installed, you will have to follow the callibration guide listed above under #3.