The TM6 form is the arrival/departure card, which foreigners are required fill out with their passport information and other personal and travel information when entering or leaving Thailand. The form was permanently cancelled for foreign travellers arriving by air since July 2022.
The six-month suspension of the TM6 form is aimed to ease immigration procedures and alleviate congestion at eight checkpoints each for entry by land and sea.
More info:
Thailand Suspends Filing of TM6 Immigration Form for Land and Sea Arrivals