I have been playing since late season 6 and IMO the matchmaking this season is by FAR the worst its ever been. This is probably in large part due to Rito's spectacular failure with positional matchmaking, and then merging all the positions together.
Not gonna include season 6 b/c i was pretty bad and only started playing ranked at the tail end of the season but s7 and s8 i played a good bit of ranked.
Season 7: Matchmaking felt pretty damn decent, I was a Jg main for all of season 7 tho i would sometimes play mid or top and obviously autofilled support. I would estimate in S7: ~40-50% of games felt pretty evenly matched, and then ~25-30% each felt one sided for my team or the enemy team (in fairness, even these one sided games were not NEARLY as stompy as this season's 1 sided games feel).
Season 8: Matchmaking felt a bit worse than season 7, but still reasonably fair, instead of 40-50% evenly matched games it was more like 30-40% while the rest were one sided one way or the other, and once again still not nearly as stompy as season 9, even if the game was one sided it still felt like there was a degree (even if small) of hope that your team could turtle and come back.
Season 9: Matchmaking feels HORRENDOUS, in 90%+ of my games it feels like I am purely a passenger: either my team is winning so hard that I could go 0-5 in lane and be way behind on CS and my team would still win around me, Or i can hard win lane, get tons of turret plates, but the rest of my team is so far behind it means nothing. In season 9 that 1 in 10 games that is actually well matched and close feels amazing, but every other game feels like you are just waiting to win/lose and it hardly matters what you do, on this winning side this is immensely boring and on the losing side absurdly frustrating. Also these 1 sided games feel 10 times more one sided than the 1 sided games in previous seasons, it just feels like the team thats winning is an unstoppable snowball, even with the increased bounty gold.
Im fairly certain the cause of most of this is positional rank matchmaking (in combo with individual role ELO's being merged together) that has just skewed player's MMR score across the board (Pre-season 9 matchmaking was actually A LOT better than it is right now). Unfortunately fixing this would be a big chore for riot as they would probably have to reset MMR/rank across the board, and because that would bring a lot of blowback from people who already got to their desired ranks it seems HIGHLY unlikely that they'll do anything about it.