the /hj tone indicator is worse than useless
As someone who has always found tone indicators confusing and irritating for exactly the reasons listed, I've been watching with some amusement as Jan Misali posted various polls which demonstrate the wildly varying interpretations of various 'obvious' uses of 'half joking'. But it wasn't until this video that I went OH. THEY'RE SLANG MASQUERADING AS AN ACCESSIBILITY TOOL.
As far as I can tell they DO make communication more accessible for many autistic etc people who happen to already hang out in circles where such slang is popular, because it is easier to talk to people using forms of communication you already know. But they are not very helpful for those of us who are used to different slang. Communities using text communication have developed slang and conventions to indicate tone and intent since well before the internet, and I can't see how tone indicators are inherently any more clear or helpful than lol,
They're sometimes more succinct, which I can see being useful for stuff like mobile messages, but I saw people earnestly suggest that everyone on a large, international discord server learn them, even though discord messages can be paragraphs long. I had to be one of several people saying "typing out '(sarcasm)' or '(this is sarcastic)' is 1000% more accessible and less work than typing '/s' and then linking to a guide to tone indicators" before the people arguing for them to be made official server policy backed down.
EDIT: Thinking about it, the extension of "it is easier to talk to people using forms of communication you already know" is that to be more accessible you should try as much as possible to only rely on words and terms etc that most people reading your words are likely to be already familiar with. As far as I can tell, people who speak moderately fluent English, regardless of dialect and culture, will generally be familiar with the concepts of sarcasm and parenthetical asides (putting extra information in round brackets). So if you add '(sarcasm)' to a sentence, they will probably be able to figure out what you mean, and if you add '(this is sarcastic)' they should definitely be able to figure it out, because that's just a standard, simple English sentence. Using tone indicators, or other subculturally specific slang like 'lmao', requires learning new terms, and is thus inherently less accessible.
At least the way I see it, maybe there's some overwhelming advantage to tone indicators I'm missing along with the OP.