Note, previous post about a person flying from KL to Bangkok with "Health Certificates".
This is not a "Covid-19" test, showing a negative result. Without this test and minimum 100K USD foreigners cannot enter Thailand.
TLDR : Answer Summary
To enter Thailand from Kuala Lumpur, foreigners must have a Health Certificate and proof of minimum health insurance coverage of 100,000 USD. A negative Covid-19 test might not be the sole requirement.