It is not a new rule. The rule has been in place and Blizzard has had the option to enforce it, although it is unclear to me if they have enforced it before.
This current ban wave is a reaction to player requests and feedback. It does not just happen for fun.
This article is a year old if that gives any idea how long ago it was last updated/posted.
Party Member Left Mythic Dungeon Before Killing Last Boss
Updated: 1 year ago
Article ID: 117469
Product:
Common Problems
* We just started a mythic plus run and a member left.
- The group wiped and one of the players left the instance.
If you encounter behavior in chat that demonstrates malicious intent to sabotage the run of your Mythic+ group, you can report the player by using the right-click option in-game.
Customer Support can’t upgrade, restore, or create keystones. Running the dungeon again with the downgraded keystone will award a new upgraded keystone, as long as finished within the time limit.
Note this is for REPEATED and numerous leaving instances. Not for leaving once in a while when a run goes bad. This is for people who sabotage keys… If it is catching too many people who are not trying to sabotage, can always adjust the limits.