Before I retired to Thailand in 1993 I was told by both a senior employee of the TRD, and by highly reputable Thai lawyers, that:-
1. Pensions, from whatever source,
were not taxable in Thailand.
2. That only the income of those
people working and earning in
Thailand was taxable.
That was over 30 years ago. Things have clearly changed since then, but to what extent is still unclear, with conflicting advice being given by different sources.