From T131177#2343273 :
This change means that it is no longer possible to copy and paste math (from Chrome at least). It might help to make the SVG fallback an actual inline image, but I can't even seem to get that to copy and paste.
I know a lot of people that copy and paste equations from Wikimedia sites (WP and WB) as a quick and easy way of displaying them in presentations and the like, and this is going to break that.
We think that this is fixed with the fix of T136709, but we are keeping this open in case there are any problems.