I'm kind of glad you asked, since other smashfags on /v/ unironically think Porkybros are shitposting due to how obscure their character is. I'll try to explain why there's so much speculation surrounding him and hopefully that will clear some shit up.
It's not just the Porky spirit that's missing, but a very impressive amount of spirits that are relating to Porky as well. Giygas/Giegue, who was the main villain of the first two Mother games is absent, Doctor Andonuts is absent, Porky's right hand man in Mother 3 named Fassad is absent, Porky's brother is absent, Porky's entire army is absent, etc. Keep in mind, a pile of puke and Ness' dad in the form of a telephone got in before any of these characters, and the aforementioned missing spirits all have a theme of being major and even reoccurring characters who are related to Porky.
Then, there's the little stuff. It's the year of the pig on the Chinese zodiac, the absolutely safe capsule (incoming spoilers for Mother 3) which Porky sealed himself in side of at the end of the game actually uses it's empty sprite instead of the sprite it uses when Porky's inside of it
and 2019 marks the 30th anniversary of Mother.
What makes his absence even more bizarre, is that Porky has an entire stage dedicated to him, and he was a major villain and boss in the Subspace Emissary. This implies that his exclusion was intentional.
In that datamine of Nintendo Online where SNES games got leaked, Earthbound/Mother 2 wasn't anywhere to be seen despite being previously released on the Wii U and 3DS eShop, and sold extremely well, being a top seller on both console. This is also the case for Mother 1/Earthbound origins. This points to the possibility of a Mother reboot or remake of some kind.
Lastly, Itoi, the creator of Mother, recently posted a survey on Famitsu asking fans what they want out of the future of Mother.