In my opinion, the League community has gotten exponentially worse in the ~5 years I've been part of this community. It's pretty much a daily occurrence for me to run into at least 1 toxic player a day, if not every 4-5 games. This year has been pretty bad for the League community, with a lot of people saying that this community is toxic more than good. I'm heavily inclined to agree with that, and I consider myself part of the DoTA and CSGO community, and they're pretty toxic.
It's against the rules to be toxic, but the community seems to be much more toxic than good from what I've seen, unless a Riot employee would like to show me otherwise. A lot of traffic goes into this game, sure, but also, from what I can at least gather, there seems to be more toxic people coming into this game rather than toxic people being ejected from the game (I'm pretty sure that a huge part of that is due to the fact that in League, it is so easy to evade bans, and even permanent bans. Just look at Tyler1, and how easy it is for him to be permanently banned even though Riot explicitly stated that he is no longer welcome to the community).
Riot hasn't really gotten strict. It feels much more like they stayed the same when it comes to banning, but have just gotten more tools in their belt. What I've noticed is that as the community grows in size, there is a positive correlation in the amount of toxicity that appears. Since League is one of the most played games on the entire planet (if not the most played. Link in the tl;dr), then it's extremely easy to see why the community has gotten toxic, especially with the age demographic (12-17, so pretty much, children and adolescents) that League is geared towards. Pair it up with those factors, and it's easy to see why the community thinks itself to be toxic, as well as others who don't consider themselves part of the League community.
Personally, I think the League community is more toxic than the Call of Duty and Battlefield communities when comparing those three together: Both have the same age demographic, but League is played much more than Call of Duty, even with all consoles and platforms combined to League's PC/Mac/Linux. I've been part of all 3 communities, and I left the Call of Duty and Battlefield community back in 2010 because there were more people sending death threats and threats of hacking me or something than actual communication.
Compare two somewhat similar communities to each other: League to Overwatch. Overwatch has become one of the most played games in the world, and the community is easily one of the least toxic communities I've been part of. The idea that people paid a lot of money to play Overwatch definitely plays a huge factor in why that community isn't bad in comparison to League. League is free-to-play, and Overwatch isn't. Blizzard also has taken a no-tolerance approach to toxicity and generally being a jerk to other people. Riot has SOME leeway for that, but not much. Both communities are large (Overwatch has approximately 20 million, while League has approximately 30 million [I'm rounding it to neat numbers]), and both have a somewhat similar age demographic, with the Overwatch community having a bit more players in the older age range. If both have similar factors, and the Overwatch community is seen as less toxic than the League community, there's a problem. Sure, people have to pay to play Overwatch, but people pay to gain access to skins in League, and skins are easily one of the most purchased items in the shop.
Basically, what I'm hinting at is that the community is the problem, not Riot, not League. The greatest common factor in why the League community thinks their own community is toxic is because of themselves. There are other gaming communities with similar traits that League has, but are considered less toxic. It's all opinion, sure, but there seems to be a collective opinion that seems to show that the League community is more toxic than good. The best way to honestly fix that is for the community to take a proactive approach to ensure that the community isn't toxic. Riot gave us the tools to do so, but those tools are extremely underutilized.
tl;dr: There is no tl;dr, only two links.
http://i.imgur.com/H4K1NgX.jpg
https://nowloading.co/posts/3916216