Crossbow is a Military Weapon in Fortnite. According to GameRevolution reports, the original 5.10 patch notes from Tuesday's newest Season five update included a note that the Crossbow had been reintroduced to Fortnite exclusively for the Playground LTM. But there was no such note inside the final version of your patch notes.
"We have some content that hasn't made it into the game yet. We're working on it and it should arrive in a short while. Sorry for the delay!" Shortly just after the update rolled out, developer Epic Games tweeted out the announcement that explains some of the planned content didn't make it into the game, however. It is at the least achievable that this unreleased content contains the Crossbow.
The Crossbow was initially introduced to Fortnite in February together with the version 2.4.2 update to help celebrate Valentine's Day along with some holiday skins:
I under no circumstances enjoyed the weapon all that significantly. It had unlimited ammo but a longer reloads time, so although it functioned as a silent sniper rifle of a sort, it wasn't all that enjoyable. Nevertheless, it was overpowered sufficient for elite players that it got pulled from the game.
There was even a Season 3 Challenge requiring players to acquire eliminations using the weapons, which proved one of the most challenging Challenges to complete that season.
Despite the fact that the Crossbow is still MIA in Fortnite, at the least the Playground LTM is back, along with the Fly Explosives LTM in which players can only use Jetpacks and explosive weapons (which includes the newly reintroduced Guided Missile).
If you are looking for some guides on weapons, visit U4GM Fortnite Weapons Guides, there are comprehensive guides to the various weapons I have written, I believe it will help you. Here is a video guide on Crossbow: