Even if Codemasters were planning on releasing these extra tracks they wouldn't announce that they were doing so until far closer to completion. From a public relations perspective it's generally best to not make anything which can be read as a promise (which on the internet is sadly anything other than outright denial) until you are relatively confident you can deliver on what you have (been percieved to have) promised.
Making overly ambitious promises too early is how you wind up with the sort of backlash No Man's Sky faced.
Ultimately that means that at this stage it's utterly irrelevant for us to be discussing whether or not these tracks will wind up being added as DLC or appearing in the 2021 release. The answer is "maybe we will, maybe we wont, but there's no way for us to know".
That is to say, please stop this pointless argument. Nobody wins here and it's not the point of this thread.
Now, returning to the original topic, if they want to add an extra track in Eastern Asia alongside Vietnam there are certainly several interesting options to consider but without having a firm knowledge of the exact situation in each of those countries it's hard to tell.
Shanghai sounds like it's most likely not going to go ahead this year as the PRC understanderbly doesn't want international sporting events which aren't in preperation for the 2022 Winter Olympics within its borders this year. With Suzuka not going ahead and Fuji presumably unlikely on top of that (beyond the fact that Fuji in November would be a brave part of the space-time continuum to host a Grand Prix to begin with) and Australia signalling that its borders will be closed until at least 2021 that leaves Sepang and Buriam as the obvious targets.
And when it comes to that I'm split. Sepang is a fine circuit with a strong track record (pun not intended) in Formula 1, while Buriam brings the prospect of yet more novelty to the calendar.