I feel that M+ is all about experience with M+ (and the affixes, how they relate to each dungeon, setups, etc), which is what RIO measures. It goes beyond just the score, although that is what people look at mostly.
I’d be surprised if it was even 5%. The RIO add-on does show how many timed runs somehow has in a bracket, which makes it easier to filter out potentially boosted players. I suppose someone could buy five or more boosts per week, every week, but it’s probably unlikely. Likewise, if someone only has 10s in time, but no 11s, 12s or higher, that might also warrant a second look.
RIO was used in Legion as well.
It works the same for everyone. PUGing end game content isn’t what I feel new players should aim for, because PUGs are generally not great places for learning. It doesn’t matter if it’s M+, rated PvP, or raids. People who organize PUGs expect experience, because they want to get the content done smoothly, quickly, and successfully.
Learning happens with friends and guild groups, and nobody is excluded from making friends or joining guilds. It does require social effort, just like in any multiplayer game.
I disagree that RIO has anything to do with elitism. It is not unreasonable that the owner of a key wants to do it with people who have the experience to do that key in time. The key is their “investment” and if they choose the group poorly, their key gets depleted. This is particularly annoying with a +10 key, because then you’re stuck with a +9.
Nobody is excluded, either, because everyone has equal access to keys and the opportunity to upgrade them. RIO only matters if someone wants to do someone else’s key, especially if the four other people are unknown entities (not friends, not guildies).
If you have your own key, you can also choose to ignore RIO, item level, classes. Would you, though, if you played with strangers whom you know nothing else about?