Looked it up, the official announcement that it would become Scala 3 happened in 2018.
https://www.scala-lang.org/blog/2018/04/19/scala-3.html
You can dig through the git history of the readme for Dotty, it's actually pretty enlightening how Odersky's perception of it has evolved over time. For example, in 2014 he referred to dotty as an experimental compiler for a dialect of scala.
https://github.com/lampepfl/dotty/commit/6a42d32181bd7b8639f...