Okay, I don't know if this is was tested before and I do know I don't see any reference in the wiki, so here I go.
So I decided to test my theory of connecting islands with bridges to the mainland, here are the results:
First, I found strange that that one side of the island could get caravans and goblin sieges. But nevermind that, I went searching the island.
I then found this spot. Only dwarfs come in here so this is genuinely an island without humans, goblins or elves. I screenshot for future reference, and went to build the bridge in the canal that you can see in the last image.
I proceeded to build the bridge, and spawned some obsidian because I didn't have enough stone because the ocean flooded the holes I made on the shores to get stone, because that was the only spot I had stone. I made the entire thing 5 tiles wide just to be sure, but by lack of materials one spot didn't have everything and it ended only one tile wide. I marked the map as a lair, just to be sure.
I build a depot on the other side to test Depot Access only to remember that one spot that is one tile wide. Oh well I abandoned the fortress.
And here's the results! I checked everywhere on the island and took a screenshot in the same place as screenshot 2, and now every civilization has access to the island. I didn't test if caravan come through but sieges do come and it civilization screen marks that you can have contact with other civilizations, so I think they will come. Next time, I must fin and updated embark anywhere mod and test a fucking long bridge.
In the futures, I will test if caravans come to this island then I will use embark anywhere or the current version equivalent of it and try and make multiple fortress bridges, so to test if a long bridge can distant islands to civilization.