When was the first time kmp was "announced"?

Depends on your definition

The very first public Kotlin preview in 2012 was a web portal with an option to compile to JS. https://blog.jetbrains.com/kotlin/2012/01/kotlin-web-demo-is-out/

Repo: • Initial commit 2010-11-08, • first commit targeting the JS platform 2011-11-03, • first "Android-related fixes" 2012-06-01, • first Native (LLVM code generation) commit 2016-07-08. Kotlin 1.1 released with JavaScript support, Coroutines 2017-03-01. Kotlin Multiplatform announced at KotlinConf 2017.

Do you have links to those commits?

Unfortunately, I don't. I had picked them from my IDE commit history.