AnotherGuy
Hero
I'm going to play Devil's Advocate on this mostly to organise my thoughts on this, so I welcome the push-back.Because to me one of the most classic forms of railroading is "DM changes the rules/way the universe works to force a specific course". It's not outrageous railroading because it's not totally inconceivable that someone would come up with a spell specifically to target the components of the Plane Shift spell, but, it's not a good look, and it frankly opens the door to a lot of spell shenanigans that isn't attractive. I also disagree that this is merely "setting up the premise of the adventure", because there's no option to say "no thanks" and there was no warning beforehand, the already-existing characters are already in the situation.
If PC travel into Undermountain for the first time and discover that their teleportation spells do not work and can thus come to the assumption they can only leave the way they came through...is that Railroading?
I am trying to understand how what @Reynard did is not the same as a location whether it be Undermountain, a Plane or anything else - which has unusual restrictions, whether the PCs were aware of it or not.
If the answer is because its established prior play, it is only established on the DM's side, it is not necessarily player-facing, so how does that matter?
Again, I'm just exploring the argument for my sake.

