This year's annual Arrowverse crossover will not include Legends of Tomorrow. Instead, the crossover, which will introduce Gotham City and Batwoman, will only incorporate Arrow, The Flash, and Supergirl.
Like the recently announced Batwoman TV series, Legends of Tomorrow began as an Arrowverse crossover event. "Heroes Join Forces" in 2015 provided the setup for the main story behind season 1 of Legends of Tomorrow. The annual event became a three-way crossover in 2016 with "Invasion!", which saw the Legends get in on the action for the first time. Then, 2017's "Crisis on Earth-X" expanded the event to all four Arrowverse shows by adding Supergirl.
EW reports that Legends of Tomorrow will be left out of this year's crossover, but no further details have been revealed. What's noteworthy about Legends' absence from the event is that so far, every crossover has been bigger than the last, and no series that was already part of the event has ever been excluded. It's likely there are story-related reasons why Legends won't connect to the upcoming crossover.
We don't know what will bring Team Arrow, Team Flash, and Supergirl to Gotham City but it's entirely possible that whatever the problem is, it won't require the help of time-travelling superheroes. The crew of the Waverider often clash with powerful enemies that threaten the timestream, so fighting crime in Gotham City may not seem like the best fit for the series.
This doesn't necessarily indicate that Legends of Tomorrow will be disconnected from the Arrowverse in the 2018-19 season. Though many of the Legends who once had strong connections to The Flash are now gone, a few of the characters, particularly Sara Lance and Ray Palmer, are still tied to the rest of the Arrowverse. Since the Arrowverse has been known to do mini-crossovers, one or more members of the crew could appear on Arrow or The Flash, just as Sara did in the season 6 finale of Arrow.
A smaller crossover event for 2018 doesn't mean that it will be smaller in scale as well. A three-hour crossover special involving Arrow, The Flash, and Supergirl could present massive repercussions for the Arrowverse, simply because of its setting. Not only will the crossover reportedly add one more series to the Arrowverse, it will open a door to a whole new corner of the DC Universe that will surely be explored for years to come. Even if Legends sits out in 2018, it could be back for a five-way crossover in the 2019 season.