I'm expecting the wall to remain present for characters that have not yet completed part 1 story quests, even after the big event to tear it down will be happened.
Probably they'll be able to reach the other side if teleporting to a player that's there but they will not be able to pass through the wall freely unless they'll have completed the specific quest to tear it down, which will likely become available as a single player quest after the big event will be concluded.
They already said content won't feel outdated because they've thought about making it coherent. These worries were expressed during the Livestream they showcased Solstice zone.
This was my big concern when the new release schedule was announced. I have multiple characters, all at different points in the story and rushing them all through the first part of the story now doesn't sound any better than playing a broken version later on.
ZOS said at the time they have a plan for this, but we don't know what that is and considering there are still dialogue problems and other anomalies as a result of One Tamriel and Morrowind which they never seem inclined to fix I'm concerned the solution will be to assure us that both parts of the Solstice story are functionally playable and all the items, XP etc. can still be earned and simply ignore any problems with the story.
IMO the ideal solution would be to use the phasing system which already exists to control what a character sees based on their progress in the story, so until you complete the quest chain you see the wall in place and can't pass through it (but can teleport to someone on the other side if you need to get there without doing the story). Ideally that will included adding a quest version of the event so you don't just skip straight from "Oh no, this impassable wall is blocking 1/2 the island!" to "Wow thanks for getting rid of that wall!" without the character actually doing anything.
More likely you can pass trough but not do the main quest. A bit like you can port to some in northern Coldharbor to dig up the void crystals to level Excavation for the treasure chest buff, you can do side quests but not the main one.I'm expecting the wall to remain present for characters that have not yet completed part 1 story quests, even after the big event to tear it down will be happened.
Probably they'll be able to reach the other side if teleporting to a player that's there but they will not be able to pass through the wall freely unless they'll have completed the specific quest to tear it down, which will likely become available as a single player quest after the big event will be concluded.
They already said content won't feel outdated because they've thought about making it coherent. These worries were expressed during the Livestream they showcased Solstice zone.