All three providers have announced and even begun to implement eSIM support.
Getting detailed information has proved challenging to me, but based on a quick scan of Thai social media (where service providers participate) it seems like TrueMove H and DTAC definitely offer eSIM support, at least in Bangkok.
AIS seems to be pushing the eSIM combined with the physical Sim2Fly combination, the SIM2Fly is used for data when roaming outside Thailand.
eSIM SIM2Fly
Whether one needs to be post-paid is unclear. Whether one needs to purchase an Xs or Xr from the provider to enable this feature is also unclear.
My advice: stop by an AIS full-service shop in a large mall, like Siam Paragon, and ask.