When blocking the end gateway on 4 sides like shown in the setup below, entities that go through the gateway that leads to this trapped gateway, will spawn inside the bedrock including the player.

Steps:

1) At an end gateway on the main island, press F3+C to copy the coordinates

2) Go through the portal and place blocks all around the gateway block like shown in setup

3) Teleport back to saved coordinates and then re-enter the gateway which will briefly suffocate the player in survival

Video
(In the video, I didn't press any movement controls after I entered the gateway and was forcefully pushed by the game)