It could be received as a free bonus after registering the game by January 31, 2019, but was available on January 29, and released for purchase on February 1st.
Ptooie is a projectile which has great priority, high damage output, good frame data and amazing ledge-trapping potential.
Down B gives Piranha Plant the longest range for melee attacks. It also gives him stun armor.
Multiple moves capable of striking opponents above Piranha Plant.
Side Special deals insane amounts of damage, making it an excellent punishing tool for stunned and paralyzed opponents. Another method would be grabbing them and keeping them within the cloud. Another strategy some use is to use the cloud to prevent the opponent seeing a charged move, like a Down B or a Side Smash.
Excellent recovery, which has a hitbox and can easily K.O. off the upper blast line if executed correctly.
Down aerial can meteor smash.
Automatically counters attempts to footstool it.
Up Smash, Up Air and Ptooie are good at catching landings.
Up Throw is a kill throw.
Side B and Down B do very good sheild damage.
Slow walking and running speed.
Attacks are somewhat slow and predictable.
Poison Breath counts as a projectile, meaning it can be pocketed and reflected.
All projectiles are generally slow and easy to avoid.
Low damage output without use of specials.
Slow and laggy aerials.
When Down B is fully charged and released, the neck is exposed and can be hit.
Poor approach options.
Neutral Attack: A leaf jab, followed by a leaf swipe, followed by a headbutt. Alternatively, the leaf swipes can be followed by a flurry of bites, similar to Corrin's flurry attack.
Forward Tilt: A bite. Pressing the attack button a second time will have Piranha Plant follow up with a second bite. It resembles the attack a Piranha Plant performs in Super Mario 64 after waking up.
Up Tilt: Flails its head upwards. Small hitbox, but does decent damage.
Down Tilt: A leaf swipe.
Dash Attack: A pot-first lunge. Lots of end lag, but good launch power.
Forward Smash: Transforms into a Spiky Piranha Plant from Super Mario Galaxy 2 and headbutts.
Up Smash: Lunges upward and bites. Appears to have two hitboxes. Small fighters like Olimar or Pichu might not be hit by this if not close enough.
Down Smash: Sweeps its pot around itself. First in the back, then in the front.
Neutral Aerial: Spins rapidly with its leaves extended, similar to Ivysaur's neutral aerial. Hits multiple times.
Forward Aerial: Swings its pot upwards.
Back Aerial: Turns around and spits out fire, akin to a Fire Piranha Plant. Good launch power, but isn't very quick, like Mii Gunners.
Up Aerial: An upward headbutt.
Down Air: A pot stomp. A Meteor Smash with quick startup compared to other Meteor Smashes.
Grabs and Throws
Grab: Reaches out with a leaf.
Pummel: Bites the opponent.
Forward Throw: Headbutts the opponent forward.
Back Throw: Chomps the opponent and flings them backwards.
Up Throw: Tosses the opponent upward as it bites them.
Down Throw: Throws the opponent on the ground and bites them twice.
Spits out a cloud of poison in front of it, dealing damage with seemingly no knockback or flinching. Piranha Plant changes its color to match the Putrid Piranha enemy from the Paper Mario series when executed.
Retreats into its pot and pauses briefly before stretching out to bite an opponent. The pot can be tilted in order to aim the attack. It possesses a form of armor while inside the pot, shrugging off various attacks. Likely a reference to Elasto-Piranhas or Stalking Piranha Plants.
To be free (albeit for a limited time to those who pre-purchased the game)
To have a showcase video on the official website
Who shares a victory fanfare with base game characters (Bowser and Bowser Jr. in this case)
To have their own Palutena's Guidance, presumably due to being developed alongside the base roster
To not have a unique animated reveal trailer; instead it is made entirely within engine.
Super Smash Bros. Ultimate is actually the first game ever to have Piranha Plant as a playable character (although Petey Piranha is playable in the Mario spinoffs). It is also the first Super Smash Bros. fighter to be a mob.
The Smash Piranha Plant makes references and shares the traits of multiple Piranha Plant subspecies.
This is one of the few depictions of a normal Piranha Plant having feet. Most depictions of the Piranha Plant are implied to be rooted to the ground, their pipe or their flower pot. The Ptooie, Stalking Piranha Plant and Elasto-Piranha Plant actually do have feet: but it's unconfirmed if they are identical to regular Piranha Plants or if they are subspecies. Ptooies and Stalking Piranha Plants are also not depicted with a flower pot: although the Elasto-Piranha Plant does have a warp pipe that it wears as pants and can still walk with its feet.
Ptooies also puff spike balls into the air in a manner similar to the Smash Piranha Plant.
Piranha Plant changes its coloration into that of a Putrid Piranha when using its poison attacks.
Piranha Plant's extremely elastic neck is possibly a reference to the Stalking Piranha Plant, the Piranha Creeper or the Elasto-Piranha Plant.
Piranha Plant's up special is a reference to the Jumping Piranha Plant: which can also use its leaves to hover.
Each of Piranha Plant's 7 alternate skins are a reference to Piranha Plant subspecies. 
The 1985 Piranha Plant skin has the classical green Warp Pipe. Half of these skins have a warp pipe instead of a pot: and these pipes use a completely different model as the pot (most noticeably missing the holes that the pot has).
The 6th color in the pallete swaps most likely references the Dry Piranha Plant from the New Super Mario Bros. series, which is a skeletal version of the famous plant. This subspecies has questioned a few as to if all Piranha Plants have bones(besides teeth, of course).
Originally in Update 2.0.0, if the player tried to play as Piranha Plant in All-Star Mode, the save data of the game could get corrupted. However it could be fixed through the Save Data Cloud Back Up functionality included with the Nintendo Switch Online Membership. This was patched in a later update.