I played competitively in Gen 3 and 4, so my experiences are about 10 years ago... But competitive battling culture can definitely be toxic. Most people are nice, giving constructive criticism and complimenting you when you have a good team/moveset/win. However, some people can be very nasty when you make a mistake during a battle, when you use a 'weak'/'outclassed' Pokemon/moveset or do something else that may be a mistake. When you get confronted with the latter too often, competitive battling can definitely be toxic. Especially if the very competitive mentality some people have doesn't fit you, things can be difficult.
I don't mind some teasing or trash talking, or cursing when you are fucked over by constant hax. That's part of the game. But there's a difference between that and just being a total dick.
In Gen 4 OU, I had this cheap annoyer team that got me quite a lot of wins. Choice Scarf Iron Head Jirachi and Air Slash / Thunder Wave Togekiss, both using Serene Grace to make it nigh impossible for opponents to attack. Add a tanky Milotic and a Spore Breloom and you get some very frustrated opponents. I will never forget the insults and even violent threats I had to deal with when using that team. I just tried to ignore the opponents and didn't reply to them, but of course it hurts when people say those things to you. I can understand that it's annoying to deal with a team using flinch hax shit, but that's not an excuse to say horrible things to a stranger on the internet.
Or the time I had this Rain Dance team in Gen 4 UU, in which I decided to have some fun with Fuck Yeah Seaking. People did not take kindly to that, being swept by a fucking Seaking. Remember that back in Gen 4, you couldn't see the opponent's team before the battle started, so they were not prepared for it.
Back then, I had (and still have) very poor mental health, and did not take the threats and insults well. I mean, even the milder things, like the harsh culture outside of flaming, were already difficult to deal with for me. I got bullied in high school, and when I got home, I had to deal with toxic competitive battling culture during what was supposed to be a fun hobby. The toxic behaviour of some (not all!!!) people in the competitive battling scene is one of the reasons why I quit competitive battling when Gen 5 started. It was also personal, just not having an over-over-overcompetitive mindset, but the way some people behave definitely did not help.
I hope I haven't offended anyone. I still believe most people are kind.
I've found myself immediately ignoring opponents the second I enter, especially when playing more defensive/stall teams. I'm gonna be real, I learned more slurs playing stall on Pokémon Showdown low ladder than I did from my racist father when he was alive.
Even not playing stall (even the "fun" offensive teams!) I've had people actively seek me out on social media to sling an insult or two at me. Even had one dude find out I'm trans on Twitter and went on some bizarre rant in my DMs with their...high school level biology. It was a good laugh, though.
Unfortunately, people will be toxic, and all you can really do is ignore them instantly on some jibberish alt...
Very sad that you have to go through that... You deserve better than transphobic shit. I've had to deal with nasty shit back in my competitive days, but this...
And what you said about stall is definitely true. I even used this annoyer team with Serene Grace Jirachi and Togetic (FlinchHax) and Poison Heal/Spore Breloom. You can imagine how people reacted to that...