I left 2 weeks ago for Australia and no Thai govt documents required. I fronted the check in counter with 2 documents - my passport and a printed out copy of a covid PCR test taken within the last 72 hrs. Negative result required of course. That's all.
I'm in quarantine right now. In Oz. Can tell you there are no cleaning services here in quarantine, never was I'd say. Who'd want to clean a room with possible virus present? Good way to spread it - I'm surprised there was in Thailand.
This counter is to the left of the normal line of immigration counters, in that area NO Photos. However it's well sign posted. I just asked an immi officer and he pointed me in the right direction. The whole airport is pretty much deserted, including immigration, so when I went into that area I was one of only a handful of people. Immigration staff outnumbered customers, so they tend to be much more human than normal, even helpful, so you'll have no problem if you just tell them you want a re-entry permit before going thru the gate.
I got a re-entry permit at the airport on my way through customs on the day of travel. No need to do anything prior to that. This happened on April 18th 2021.
Just to complete the picture, I came to BKK Thursday from Korat, visited Dr Donna in the arvo, made an appointment for 8am Friday. Did that and by 7pm Friday had my PCR test result (negative fortunately). Saturday morning had the document printed at a photoshop and bobsyeruncle. All set for the flight today. 😊