Dotty / Scala 3 was so far not fully subject to the SIP process, although it tries to track SIPs in its implementation (last one is SIP 35) and has proposed SIPs on its own (e.g. trait parameters).
As its definition solidifies that will probably change in the future. Regarding
then, we have much more freedom to introduce this in Dotty because we assume existence of a rewrite tool. Without such a rewrite tool, migration is more complicated. That’s why I did not want to pursue this further in the Scala 2 context and was happy to close SIP 12.
That said, and in view of LPTK’s comment in the corresponding Reddit thread https://www.reddit.com/r/scala/comments/80rb2u/the_state_of_then_by_eed3si9n/
I would be in favor of deprecating do-while now, because it would make the syntax more regular and would make migration easier.