I know what u mean, i always load up na.op.gg during loading screen to see what ranks people on enemy are and in which lanes. like just to be sure our top bronze isn't against their top plat, or to find our plat adc is against their bot bronze support, etc. Just to make myself smile or cringe though often it doesn't work out as you would expect. I was super high comfort mid pick which counters traditionally and as i play it mid assassin who was listed as bronze 4 and next thing you know she murdered everyone 100x and game was over. :( seen a diamond mid lose to a silver. So it's more that I check it for kicks, and unsure if I should tell our silver solo lane that the guy he is facing is 74% winrate playing on his main, plat cause may just screw up his confidence rather than have him play more conservative.
Usually when i see that it tells a person is in promos it seems to be off a game or so. I'll ask someone on my team and they'll say oh that was a few games ago. It seems to me that the promos diagram is often old info. Maybe not always or only in the few cases i've checked.
Usually you don't wanna tell someone in champ select that you are in promos but one of my games a guy on my team did say that but then he super hard carried the game himself making sure nothing could stop him from winning. I was fid v darius top which made him wince when i selected it, but he was nice guy and hecarim jungle crapped all over the darius just in case I needed him. Hard to turn down a jungler's offers of assistance :P
P.S. I made a run at end of season 6 at gold and got to silver promos to gold so many times just couldn't get through, you often have someone having a crappy game, but sometimes it's not intentional, I've certainly had my own as I mentioned at the very top :)