The summary of the ChangeLog/commit history is in the changes
file in each Tcl source distribution. The one from Tcl 8.6.1 can be downloaded from here (way too long to include!) and the parts that you are interested in are toward the bottom. (It goes back to about Tcl 3.0, so there's a lot of history in there.)
Summaries of recent releases are on the Tcler's Wiki.