Exiled from Continuity: With some exceptions. When Swamp Thing officially went to Vertigo Comics, it was no longer considered continuity, for all intents and purposes. Once the series ended, he would make sporadic appearances here and there, often with a Shout-Out to the series.
DC has recently announced plans to bring Swamp Thing and other Vertigo characters back into the DCU.
Also Neil Gaiman's run, which would have followed Veitch's. He pulled out of in protest after Veitch's "Jesus" idea was vetoed, but he's said that it would have built up into a crossover of all DC's plant-themed characters.
Mark Millar's run was supposed to have climaxed with Swamp Thing #175, but he ran out of Grant Morrison-written scripts to slap his name on, forcing the massive storyline he wrote to be prematurely ended and the series cancelled with issue #171.
A Swamp Thing/Poison Ivy story was written in the mid-90s for publication was shelved due to to conflict between Vertigo and Batman editors. The story would have revealed that Alec Holland and Poison Ivy had an affair and that Ivy spent years bleeding Alec Holland dry financially and forcing him to steal chemicals for Ivy to use in her criminal capers, by threatening to out the affair to his wife Linda. Years later, Ivy and Swamp Thing would meet for the first time as Ivy attempts to gain plant-based elemental power and Swamp Thing would unleash unspeakable vengeance against Ivy for the years of torture Alec endured; when she uses her power to turn herself into an elemental, Swamp Thing would use his own powers to override Ivy's usage of her own new power, turning her into a tree THEN stripping her of the ability to turn herself back via destroying the artifact she used to gain her powers, trapping her in an And I Must Scream hell. The story never got published because the Batman editors got tipped off that the Vertigo editors not only would not let them undo the story so Ivy could appear in future Batman stories, but they would attempt to STRIP Batman editors and pretty much ANY OTHER DC WRITER of any chance to use Ivy again, by claiming that since she appeared in a Vertigo book, she was now irreversibly Vertigo property.
Actor Swap: Original plan was to have Ray Wise as both Dr. Holland and Swamp Thing, with Dick Durock as his stunt double. But the two men ended up looking drastically different from each other as Swamp Thing, so Wise plays Holland and Durock plays Swamp Thing.