Autistic children cannot be disciplined the same way that NT children can
Is it doing them a disservice not to discipline them?
Saying that autistic children cannot be disciplined in the same way as a NT child doesn't mean not disciplining them at all, it just means you have to take a different approach because what works for a NT child won't necessarily work for an autistic child.
For example, I have an autistic DS and a neurotypical DD. If DD misbehaves then I would correct her there and then with "no, you don't do that because xxxx" and then if it was something that needed consequences then I'd try make the punishment fit the crime. This might be confiscating her tablet or other belongings, leaving a place and going straight home, docking pocket money, grounding her, sending her to her room, etc. If DS misbehaves and he's in an agitated state (which won't necessarily look like agitation, he can be eerily calm when he's really very stressed) then correcting him there and then can trigger him off so removing him from the situation and calming him down is often the priority before any sort of correction can take place. We have to think carefully what to say and how to phrase it as he doesn't have the same level of reasoning and processing as a NT child. Certain consequences don't work on him. He is not bothered about being grounded and actually likes it, same for being sent to his room. He uses his tablet to help him regulate - noise cancelling headphones on, videos of factory machines doing repetitive actions to calming music, head down and shut out all other sensory input for a while - so removing it can be counterproductive and cause worse behaviour. We tend to have to be creative in the consequences for him. He's very clever too. If he has reached his limit then he will deliberately wind up his siblings and incite them to misbehave to try and make me say "that's it, we're going home". Sometimes he misbehaves and it is not due to being autistic, it's because he's a child and children are sometimes naughty. Docking his pocket money works rather well in those situations.
How come there weren't autistic kids in the supermarkets years ago having meltdowns like you see now?
There were there, they've always been there, but they weren't autistic they were just naughty/boisterous/a handful as diagnosis then wasn't as developed as it is now. And taking children everywhere with you is a relatively new phenomenon as is doing a week's worth of shopping/several days worth of shopping in one go. I rarely went to the supermarket with my mum as a child because she would either do the shopping when I was at school, leave us playing out for half an hour while she nipped to the shops, or left us with a neighbour or my grandma. My brother was a boisterous child so she never, ever took him even when she took the rest of us, she'd learned that he couldn't cope with the shops so spared them both the stress of it. And let's not forget that children who were diagnosed with autism back then were usually at the more extreme end of things and parents were encouraged to institutionalise them or place them in residential schools.