Another parent here with children who use tech in public. By that I mean on trains, aeroplanes, in restaurants, at the bus stop, in the shoe shop, at the playground basically wherever and whenever it is needed to avert a meltdown.
I have 3 dc, all with ASD. Only the eldest, with severe autism, needs it (and therefore gets it) all the time - waiting ability is zero, especially this week after 6 weeks of holiday (therefore break in routine) and back-to-school looming ( a Good Thing, but stressful as new class, new children, new teachers) next week so anxiety is sky high. She is 13, 5'7, and a meltdown is utterly uncontainable and is likely to result in injury (usually me, but others have been involved). The younger two manage reasonably well, but tbh, yes there are times when they will be 'plugged in' too. Usually after a hellish day, where they have coped with a shitload of challenges from their sister, or managed disappointment after disappointment that we cannot do X, go to Y, stay at Z, because their sister can't cope - sometimes some downtime is needed for everyone.
None of mine will tolerate headphones. Sensory issues. And so the volume is on, but low. It is, of course, audible, but not to a point above ordinary conversation levels. And not generally for games.
TBH, I think dd1 using an iPad with low volume is vastly preferable to the period where she would sing (loudly and enthusiastically) continually. She sings really well, but impromptu concerts aren't what the general public are after, I found. An iPad on low is also preferable to the time when she would recite whole books, over and over. Several passengers complained on the transatlantic flight when she quoted the Gruffalo over and over and over for about 4 hours (not particularly loudly, but again, audible) - wtf was I supposed to do, gag her? It was her coping mechanism.
I actually find ds' constant tapping of cutlery and crockery far more annoying (again a coping mechanism), or dd2's incessant stream of gabble relating to trying to categorise things (anything at all, could be related to a film we have just seen, could be ice cream flavours, could be glass sizes, whatever she matches onto) - she barely pauses for breath!