Where does it state that on the tax return form? The tax return form only has spaces for "assessable income" which the pension is not, so there's actually nowhere to put it.
That's my position. Fifteen years here, the rule change (delaying remittance until a later year) doesn't affect me, as I've always transferred pension money the same time I received it. Never had to get tax ID or file a return. As many countrys' DTAs state pensions only taxable in "country of residence" - I retain Australian Tax Residency - it wouldn't be taxable anyway. So my head is very firmly in the sand and will remain there until the Thai tax office issues clear instructions circulated amongst expat communities. There's a lot of doddery old farts out there never use the internet and have no idea of these changes. I can also see a clear "amnesty" for a year or so, until everything is settled and communicated.
All income won't be taxed. It's dependent on each individual circumstance. A person might be a tax resident in another country which would take precedence or might only transfer pension money which may be non-taxable in Thailand. Too many variables
It's a five minute stroll to my closest PO, postage and return with EMS about 70 baht, two or three photocopies a few baht. Box ticked. I laugh when I read about people spending half a day at immigration just for a check-in..