On raider.io there are 34.395 guilds that have done heroic and 14.960 that have done mythic Uldir. If there are 20 people in a raid team, that means at least 6,8 million people have done heroic raids and 2,9 million people have done mythic raids. So clearly there's still plenty of people still doing this content that at the same time is exclusive.

As for mythic dungeons, there have been 13,2 million registered runs, each run meaning five people have done that run. I couldn't find character rankings yet, I think they recently added that feature.

Now I do realise a lot of people (like me) unsubbed from BfA, but it definitely not because of LFR.