Well, the raider io addon has the means to implement a function to note on their score boards when someone left a group before the dungeon was completed.
I did think of making an addon which would record and display this, but raider io could add the feature to their addon for far less trouble.
Anyway, you were punished for someone else’s actions and they face zero consequences. This is why I have made the suggestion that the Keystone system should be changed.
Check out the topic where I wrote the suggestion if you feel like it.
https://eu.forums.blizzard.com/en/wow/t/a-new-improved-system-for-mythic-keystones/