I have not experienced toxicity in URF, but have heard lots about it.
I keep seeing posts about how people are playing URF and how it was "way better last year" "I picked X off meta champion and my team flamed me all game"
I unfortunately missed URF last year because my LoL client wasn't working, but I have played about 7 games so far this time around, and I haven't seen ANY toxicity or flaming. I even went AP ashe, and went something stupid like 3-15 and my team didn't even mention it. The only "flame" I got was when mid game I was asked "why ap ashe?," but that changed quickly when I ulted half of the enemy nids health, and was setting up plays in other lanes with my ult.
Not saying that flaming and toxicity isn't happening, but I personally haven't had any issues at all, and I feel that it is ok to experiment with weird builds or champions.
Edit
Thanks to LeagueSrsBsns
I wasnt here last year but this is what I go through "maokai is useless urf champ" "report for playing useless champ" "report for feeding" "didnt fking ban hecarim" "/all told them to ban hecarim but NOOOO" "afk, no hecarim ban" "hecarim is so OP, why didnt you ban him" "I remember when hecarim was instabanned last year"
See these are the things that I have heard about but I haven't personally experienced