For example, changing the way something as fundamental as regions work
in a very mature editor is just a bad idea. A really really bad idea.
By all means, define some other type of region and let that have other
semantics, or, let users opt in to incompatible regions by having them
set a variable in their .emacs.
Like Eli, I really wonder what this means. I've been using Emacs for
about 25 years, and I've not seen any change in the way regions work.
Perhaps he is talking about transient-mark-mode?