The TC39 process is well-documented, you can check out the stages and guidance for providing input here: https://tc39.es/process-document/

The do expression proposal is currently stage 1, so its very early. You can check out the issue tracker there to see some of the related discussion. Standards work can be deceptively hard, even for simple things. I think the do expression proposal is a good example of that.

Edit: forgot to link to the issue tracker I referenced above https://github.com/tc39/proposal-do-expressions/issues