Is Wise transfer no longer an option for funding the 800,000 baht requirement for a Non-O retirement visa in Thailand?

Where you would have to prove the funds came from abroad were if you were trying to move to the 65k monthly method and would have to prove each monthly transfer came from abroad. As you have been told, Wise have a clear workaround for this when you are submitting the payment although Swift is also an option but a little more expensive usually. Remitly, Revolut etc. also likely more cost effective options.

Swift is easy

If that right which I doubt, just put the reason for transferring the money is to buy a property

If your transfers with Wise are not showing up as International Transfers despite selecting "funds for long stay", wise has a section on this guide covering the topic on how to solve that and get proof of international transfer.

35/guide-to-thb-transfers

Wise is fine

As mentioned

There is a description for the transfer that must be completed - choose

“ funds for long stay “

In case you ENTERED Thailand on the 90-days Non-Im-O Retirement Visa, for the application to the 1-year Extension of (the) Temporary Stay Permit based on Retirement/ being over 50" on Immigration in Thailand, you do not have to prove that the deposit came from abroad . . . . . .This proof is only required if you did a touristic entry and apply on Immigration for the "change of visa type" to the initial 90-days Non-Imm-O Retirement Visa. In this case you must prove that you already have a Thai bank account with 800,000 THB already sitting in it, and in this case you must show that the deposit came from abroad. WISE offers a menu in which you can select an option that will show that you remitted the money from abroad

UK bank to Thai bank is ok, a slightly hire fee but the same exchange rates. Send it in Sterling and convert to Baht in Thailand