Sonic was the biggest Smash request period and was from a game that could not support DLC. The datamining of Smash's base game showed us Plant's Guidance too alongside the DLC placeholder, placeholders for DLC is a common tactic in game development too, so why was Plant unique in that regard? Not to mention Plant was not finished until post release. Do you know how I know this? Because Plant wasn't day 1 DLC, which again, would suggest they planned Plant as DLC, only started working on them during late development when they knew who was going to be in the fighter pass and decided Plant would be the only DLC character with a Guidance for some reason. I don't even get the hostility as I said this as neutral as possible, there's more evidence in favour of Plant being for base than against it
Let's pile up the evidence on my side, shall we?
CPSIA amiibo codes for base game characters are grouped by franchise. For example, Pichu, Squirtle, Ivysaur, Pokemon Trainer and Incineroar are all grouped together despite having different release announcements. Same applies for Ridley and Dark Samus. However, Plant is the sole exception to this, being after every other base game newcomer rather than being grouped after Daisy, who was the very first Ultimate amiibo code-wise. Why would Plant be this weird exception to an otherwise 100% consistent pattern if he was a base game concept?
Hell, the very
idea that Sakurai charges for Plant is also another good argument because charging us for something that should've been a base game character is just incredibly scummy and very unlike how Sakurai tried so hard to please everyone with the sheer insanity that is Ultimate's roster. If he was a base game character that didn't make it to release, he'd be either free DLC or completely scrapped.
Nipper Plant evolving into Petey also makes no freaking sense. What makes more sense is Piranha Plant evolving into him, so this tells me they changed a specific spirit during development to make Plant a fighter, and unless it's clones, characters in a Smash roster
always follows the original plan, so this tells me Plant was thought of much later in development (specifically when Spirits were being implimented and tested). The very idea that Sakurai thought of Plant
during development rather than being on the initial plan like any non-clone of any Smash game ever doesn't mean he was a base game concept at all.
All this tells me is that he was thought of as an early purchase bonus later in development rather than being on Sakurai's mind since December 2015. Especially the fact that the amiibo isn't grouped with Daisy like any other franchise that added more than one amiibo for Ultimate, that's just a dead giveaway that he was always meant to be DLC.