Lately, It has been published on some Official Websites that the Transit in Bangkok became allowed for Tourists Joining the Phuket Sandbox Model. However, Not much details have been Published on how this will be implemented and on whether there are further rules or restrictions about that or not.
First of All, yes it’s allowed, but with certain rules. (Sealed Route Flight/Airline Only)

Customers have to Purchase the ticket from the International Airline including this route. It cannot be 2 Separate tickets, one for arriving Bangkok from Overseas and another one for flying domestically from Bangkok to Phuket. It has to be the same ticket including Both Routes.

Many Travelers have already booked their Flights to Bangkok since long time, and they were waiting for this update to happen, so they can take the chance to enter the country through the Phuket Sandbox Model.
Unfortunately, for these Travelers, it’s not just that simple to buy a Ticket from Bangkok to Phuket. Travelers must Contact their Airlines to Add the Domestic Flight Sector (Bangkok to Phuket) to their Ticket, so it can become Eligible to be used.

Remark: The Connected Domestic Flight from Bangkok to Phuket can be by Different Airline, it depends on Which Domestic Airline the International Airline will be Code Share with.
In case of Connecting, guests will enter VIA Gate E10 and will take their Domestic flights form Gate E9. Same thing is applied for the Samui Plus Model.

This Sealed Route is to ensure that the incoming international Passenger Connecting their flights to Phuket will not go outside of the Designated Areas before taking their Domestic flights. In addition to that is the Luggage Claim and Passing the Immigration.

That’s according to the AOT (Airports of Thailand) on 30 September 2021, and Subject To change.