Currently I was in KL filling out TDAC first time: NO RED ASTERISK BY THE ONWARD TICKET FIELD. Verified by travel agent I use in CM: “not required.“
At some point in the future this may be activated, but now for use with exit flight info; if available. TDAC has a “revise form” option if flight data changes. They do match real tickets info.
I have had a Bangkok Bank acct since 2017 and send money to cover my rent and electricity for a studio I rent by the year but only come to CM as a tourist now.
Does this affect me if you know? I am not in Thailand now but will be visiting soon.
I have never needed one from Thailand coming or going, but in Singapore two years ago, I was forced to buy a return ticket to Singapore.  The attendant at the airport claimed that was a rule.
I chose a cheap SCOOT ticket and after landing in Thailand canceled that return ticket.