Oh come on, not the same old tired clichés.
Parenting has changed but that's not a bad thing. And it's not changed that much. IMO you have four "evolutions" of parenting (or behaviour management):
Scary punishment based (hitting, aggressive shouting)
Calm punishment based with some reward (e.g. Supernanny)
Positive parenting - more reward/encouragement, very little punishment
Collaborative/problem solving type - lagging skills/behaviour as communication model
Most (good) parents these days are operating in steps 2 or 3. I'm in two minds about whether step 4 will ever become mainstream. Step 3 works really really well, absolutely well enough for the majority of people and tends to have good results for children too. Step 4 is really time and energy consuming if you do it right, so it's not worth it for most people to make the jump from step 3.
Schools are not parents and they are not parenting, so the four "parenting styles" probably don't perfectly map on but as methods of behaviour management, to me it seems like they are mainly operating in step 2, but hoping that the punishments will have the same effect as step 1. That doesn't work, IME. The reason that you can use calmer punishments for step 2 is because it's balanced out by relationship and because it's a parenting style aimed at children who are neurotypical and not traumatised. It only takes one child with disruptive behaviour to disrupt a class, and there are neurodivergent children and (unfortunately) traumatised children in every class. Step 1 type behaviour management seems to work to an extent on these children although it's hugely damaging for them, so we shouldn't go back to that. I'm not sure step 3 works well without the parental relationship part either. Step 4 does work in theory in a school environment, and works with all children including neurodivergent and traumatised, but it's time and energy consuming and I don't think it's feasible the way schools are currently set up. You'd need much smaller classes for one thing. Maybe that's the problem - we're still operating with the parameters that worked with discipline management style 1 and not adapting to the fact we find that distasteful now.
I agree that special schools closing in favour of "inclusion" was a terrible idea. Not that children should be segregated, but inclusion is implemented so terribly that it is basically pointless. Either open those schools again or do inclusion properly. (Like that will happen.)
Are there more traumatised children now? I don't know if there are more or less than in the past. Certainly there are a lot. CAMHS seems totally overwhelmed and unable to operate properly. COVID has exacerbated social problems. Social services seem totally overwhelmed and unable to operate properly. I think it's possible that children who are having a hard time are maybe less able to completely slip away (consistently truant, be ignored by authority/adults etc) like they might have done in the 80s/90s, but they also aren't getting their needs met within the system and that comes out in behaviour in school.
Is parenting worse? Is gentle parenting to blame? I would argue gentle parenting done properly is basically steps 3 and 4. A lot of people think they are doing gentle parenting when they are just being permissive, though, which isn't very helpful. But there have always been permissive/ineffectual parents. Just as there always has been and always will be violent parents and neglectful parents. Perhaps it's more socially acceptable now if you claim that you're gentle parenting. Or maybe people just feel less able to comment on others' parenting or "interfere" with other people's parenting.