Being ignored is not the key factor because she could still make that lightspeed jump, and also you could come up with (again) dozens of ways to ambush people or come at them from differing angles to prevent being fired upon. This tactic could have been used on either Death Star, on the ion canon fried star destroyers in Empire, on the droid control ship in Phantom Menace, etc., etc., etc.
Let me put it this way: it took less than forty years for people to go from heavier-than-air flight to kamikaze attacks. Lightspeed existing as it does in Star Wars for howevermany years, somebody before Holdo should have thought of this and employed it.
It's not really a plot hole with the other movies - they didn't invent this. It's a question-raising bit of writing from The Last Jedi.
At the very least, they should have used one (or both) of the other ships in The Last Jedi to ram the imperials chasing them. They could have used the escape pods that Finn and Rose used, too. They could have sent a legion of small-large projectiles at the enemy ships and turned their whole fleet into cosmic dust. But they didn't.
And, again, I do not see any evidence that the tactic wouldn't work even in the thick of combat. Poe demonstrates that you can hyperjump up to an enemy ship while they are watching you. Han Solo flies through a planetary shield in The Force Awakens. The A-Wing that crashes through the super-star destroyer's bridge does so in the thick of combat. If he can do that in regular-speed, he can do it at a much, much faster speed.
There is no reason this shouldn't work.
In ROTJ. The imperial fleet engage the rebels with the intention of keeping them from escaping. If the rebel ships were able to jump to hyperspace in spite of all this attention then that order would have been completely pointless.
So there are reasons it can't happen they can't succesfully jump in those conditions. The condtions with the Supermacy and the Raddus, Hux commanded his officers to completely ignore the resistance ship. So they were different conditions which allowed Holdo to do what she did.
Well, I'd argue that the Imperial fleet in Return of the Jedi could stop the Rebels from escaping because they would have destroyed themselves trying to jump through the ships, but not that they would have obliterated the Imperial fleet while doing so.
That's why I have a problem with what The Last Jedi does. Because it changes the dynamic of what happens when a ship hits another ship at hyperspeed. It seemed to be implied (by Han in Star Wars) that you couldn't just fly anywhere. But it was only ever shown in The Last Jedi that you can shatter other ships by doing that.
It looked to me that it was the energy from the actually entry to hyperspace being refracted as it interacted with the lead ship which caused the knock on effects.