It seems that there was a plan to include this feature in the language, but I cannot find anything in documentation about it. Does Kotlin support expression trees? Is this feature still on the roadmap?
No, Currently Kotlin (and for 1.0) does not support expression trees. There has been similar talk in the Kotlin chat on Slack that something might be coming in the future after 1.0. It is best if you go there, to the channel #language-proposals and discuss future features there that are under discussion and have no concrete answer. Kotlin Slack.