Would someone be able to explain why a permanent chat ban or a chat ban lasting a full season or longer isn't considered an option for when a player is perma-banned for excessive typing, verbal abuse, etc?
Because of two major reasons;
- Permanent/Extensively long Chat Restrictions do not work. Riot tried using indefinitely-stacking Chat Restrictions a long while ago, and they didn't reap any good results.
Players under longer-scale restrictions either used their limited amount of chat for extreme vulgarity and harassment - which is exactly why they got the chat restriction in the first place - or worse, resorted to trolling and intentionally feeding to "prove a point" to their teammates. The uptick in gameplay toxicity is a wholly unwelcome consequence of applying a chat-related punishment, and simply permanently removing the player's ability to chat doesn't remove their ability to be toxic.
- There needs to be tangible consequences for continually breaking the rules. Whenever someone makes an account for League of Legends, they agree to abide by the Code of Conduct and the Summoner's Code. These are fair and simple behavioral guidelines and rules that are easy to follow - so much so that they can easily be summed up with "don't be a jerk".
Of course, some people will falter and break the rules by incident with the occasional instance of flame or one-off outbursts. These players usually rectify their behavior pretty quickly after the first punishment, and seldom need to be punished again. But some people will flout the rules, disrespect the game, and ignore warnings and punishments and continue to break the rules; these players need a punishment more severe than a simple chat restriction.
And, as I so often say; "If you can't be bothered to play by the rules, Riot can't be bothered to have you playing at all". Players are given ample warnings and opportunities to reform their behavior, so breaking the rules again and again has to merit some severe punishment. Which is why Riot employs permanent bans; if someone is so willing and eager to continue breaking the rules after being chat restricted twice, and being forced to leave the game for 14 days, with that punishment also coming with a warning that any further misbehavior will result in a permanent suspension, it's clear that the player does not care about the rules or the game, and does not deserve to play it further.
Those are the two biggest reasons that permanent account bans are used and permanent chat restrictions/bans aren't; perma-chat bans don't work (they simply result in an uptick of gameplay-related toxicity), and players who continuously ignore and disrespect the rules need to be given a harsh, final punishment that prevents them from breaking the rules again.
...but with the ping system I feel like you don't lose too much by losing your ability to type if you aren't using it for anything but flame anyway.
Frankly, someone who reaches the point of being permabanned for flame is just as likely to abuse the ping system as they do the chat system.
