Is there a way to get the number of bosses killed within a Instance?

I am developing an add-on that would greatly benefit from knowing how many bosses are dead in the instance that player is currently at. I tried to figure out a way to do it and tested many things but I couldn't make it work. The API function GetInstanceLockTimeRemaining (https://wowwiki.fandom.com/wiki/API_...kTimeRemaining) returns "encountersComplete: Number - Number of bosses already dead in the instance." but this function just works if the INSTANCE_LOCK frame is show (https://www.townlong-yak.com/framexm...Parent.lua#872), so here it comes:

Is there a way to query the number of bosses killed within a Instance?

BTW that's my first post here on the forum, so please forgive me if I posted on the wrong section, thank you.