Daily temperature records only go back to the 1800s in most countries. In Thailand not even that long.
Temperature records of the last 74 years
"The hottest temperature measured from 1949 to January 2023 was reported by the Nakhon Sawan weather station. In May 2016, the record temperature of 43.7 °C was reported here. The hottest summer from July to September, based on all 15 weather stations in Thailand , was recorded in 1987 with an average temperature of 28.9 °C"
https://www.worlddata.info/asia/thailand/climate.php
So the hottest summer was 1987. 36 years ago.