To create a wiki post you simply mark an existing post as wiki-editable.
All done! Your post is now editable by everyone.
The UI has changed now. When you click on the ellipsis button, you next need to click on the spanner button to get a list of Admin tasks, the last of which is "Make Wiki".
Does this mean users can no longer mark their own posts as wiki?
No. Users are now granted the wrench. The only tool in there is "Make Wiki".