Any participation you had in the Private Topic before you removed yourself from it, will remain intact with that Private Topic. The only difference is that you've removed yourself from further participation. This is why the "[User] has removed themself from this conversation." is important. It tells all remaining participants that you stopped being involved with the Private Topic at that point.
I thought on past versions the delete button said something like "remove me from topic"
I could be thinking of different forum software though.
No, you are absolutely correct. It previously did say that. But that message was confusing to users who were familiar with every other forum software currently out there. So the wording on the button was updated. The feature continues to function as it did before.