I'm gonna pick out a few of the points from your list here that either need correcting or further comments, and I'll get to your TL;DR afterwards.
Any form of swearing even if not aimed at an individual is capable of getting you a chat restriction
This is not true. You can swear all you want within reason, and as long as you're not leveling those swears to insult others, you won't get chat restricted for it.
I've exclaimed "fuck" or "shit" after missing cross-map ults (Trueshot Barrage, Enchanted Crystal Arrow, Super Mega Death Rocket!, etc.), and have said "damn" or "ah, shit" after bad teamfights/scrums/etc. - and I have yet to receive a chat restriction.
Swearing is fine. Swearing at people is not.
Typing too many sentences (if it irritates another player) even if aimed at no one can get you a chat restriction
I'm gonna call this conjecture. As far as I've seen, there's only very little correlation between chat log length and the likelihood of being chat restricted.
Of course, it is against the rules to spam chat and disrupt the game flow, I would still take your statement about "typing too many sentences" with a grain of salt. I have yet to see chat log length alone lead to a punishment, so unless you're spamming "?" or other stuff like that, I would posit that it's the content of your logs that matter more than the length of them.
Anything near the topic of racism including "racist" and mention of specific colors is likely to strike you immediately with full chat restriction or instant permanent ban. This includes names. "some black guy"
I'm skeptical, but at the same time, there's no real reason to be discussing race/skin color in a match of League, so I'd say the point is moot.
Typing too many sentences + swearing even w/o directing it at anyone is more likely to get you punished.
This is just points 1 & 2 put together.
Lastly, we questioned support in multiple tickets ------- the resounding "standard" that seems to be held is this quote from support when asked about banter
"I wouldn't type anything outside of telling your teammates gj or calling an objective because anything you say may be misconstrued :/"
So if someone reports you for chat and you were using chat there is a percentage chance you will get punished. IE save your typing for just the game.
To start off, let's get this out of the way, since it's probably one of the biggest misconceptions I've seen on the PB boards:
Banter
noun
the playful and friendly exchange of teasing remarks.
verb
talk or exchange remarks in a good-humored teasing way.
Banter is NOT synonymous with toxicity.
Banter is saying things like "nice flash" or "not even close" or jabbing about how you've become a raid boss after being 3+man ganked. If you're getting punished for "banter", then it's 100% not banter.
So, with that out of the way, let's look at that quote from Riot Support:
I wouldn't type anything outside of telling your teammates gj or calling an objective because anything you say may be misconstrued :/
This is, on its own merits, and in response to the question of "banter" (which, as I've mentioned, is often misconstrued for toxicity), solid advice.
The two most bare-bones uses for chat are complimenting/having friendly chatter with your teammates, and strategizing/working through the gamestates. There is, naturally, a realm of other things you can use the chat for, but those two are the safest uses that the chat has.
Judging from that, I'm fairly certain that the Support Agent gave that response so as to avoid dealing with the "banter" confusion. Since a fair few people seem to not be aware that banter is light-hearted and well-meaning, it's altogether safer to just encourage people to use the most basic, safest forms of chat so as to avoid punishment.
So if someone reports you for chat and you were using chat there is a percentage chance you will get punished.
Using an RNG-based punishment system seems a little...Bad, don't you think? Why would Riot implement a system that punishes people based on RNG, when that could wholly lead to the possibility of someone with a 99% chance to get punished (hardcore flamer) getting to go off scott-free, and have someone with a 1% chance (early gg) getting punished immediately for an inanely modest offense?
There is no reason to assume that there's a percentage chance of getting punished for anything. Such a system would be flawed beyond compare, so it's safer altogether to assume that there is a rigid and logical system behind punishments that does not deal in RNG.
TL:DR --- Riot should tell people openly "only use your chat for the game or compliments--- straying outside those guidelines risks punishment"
There's a few pretty big issues with this proposal, first and foremost, people would assume from it that Riot intends to police chat and keep people from saying anything outside of what they specifically want to be said. And that's just not true.
Second, even if they did make a public statement - what good would it do? I mean, they have the rules up for everyone to read, and people either ignore it or don't understand, or deliberately misconstrue and twist the words to suit their view of things...
So, if Riot warned people to only use chat for complimenting teammates and strategizing around the gamestate, how many people do you wager would listen?
And, of course, there's the fact that "straying outside those two domains risks punishment" is generally not true. You can stray pretty far away from complimenting teammates and strategy without getting punished. Discussing skin lines, rune choices (although that's kinda counted as strategizing), Champions, lore, etc.
So telling players "only compliment or do strategy - otherwise you may get punished" would only serve to foster ill-founded paranoia.