nothing changed Jan 1 2024 other than a loophole regarding remitances was closed. The current administration is considering reopening it, if they survive long enough. I can find no evidence that the requirement to register has ever been enforced against expat retirees. Their pensions are not taxable, and the tax office do not seem to want the hassle.
When l used the monthly income method, l was always required to show the source of my money in the UK. I used to attach my yearly P60, and that was sufficient for Korat immigration.