If you go too far, you miss out on Chrysalid's Memento, used to obtain a golden Seed or two in Roderika's quest, the trigger for this is possibly simply just going past Godrick's Castle.
If you visit Revengers shack and get invaded by Edgar, this skips right to the end of Edgar's/Irina's questline, with Irina dead and her father grief-stricken. I'm not sure if you actually miss out on any items by doing this (apart from Irina's letter which is obviously just a quest item). I am sure Grafted Greatsword is still available for example.
Part of Rya's quest involves taking you to the manor when you visit Altus for the first time so her quest might skip ahead slightly so she's at the manor, I'm not sure how to trigger this exactly.
It might no longer be possible to find Rogier at Stormveil or in Raya Lucaria (I believe I've also seen him at the Church of the Cuckoo grace). This is OK, because he still sells things at roundtable hold.
Corhyn will express a desire to move on from the round table but this is a natural part of his quest.
Ensha will also invade you and die at roundtable hold, at about this point.
You might see/fight Margit's image outside Leyndell before you fight him in Stormveil, but again, nothing serious.
The Radahn festival will appear, which prevents you from obtaining the Red-Hot Whetblade or fighting the crucible knight / leonine boss until Radahn is defeated.
Iron Fist Alexander will move to the Radahn festival and you will miss an exalted flesh.
This is not a complete list. I too would be interested to know if there are more serious consequences with other NPCs, for example I don't know what happens with the blackguard. Also I may be wrong on some points.
The most serious ways you can accidentally kill a quest or lock yourself out of getting an item is not by going on the plateau, but by getting the finger slayer blade or by killing Rykard.