Being the madman I am, I have decided to compile all of the Kirby based Sakurai bias evidence I could find into one big list. Also, I decided to categorize everything based on fighters, Spirits, Music etc. Feel free to add anything I missed!
Fighters- There are only three Kirby fighters, Kirby, Dedede, and Meta Knight, all of which are from Sakurai games. Kirby seems to be based on his super star design, which explains why his eyes are so small in this game. Dedede's design is based on his classic look, which is why he appears much bigger and goofier in Smash than he does in the modern Kirby games.
Music- The only non Sakurai affiliated remixes in Smash are 02 and the Squeak Squad theme, which is essentially nothing compared to the countless remixes from Adventure and Super Star. Every modern song is lazily ripped straight from the source games.
Stages- Dreamland, Green Greens, Fountain of Dreams, Halberd, Dreamland GB, and Great Cave Offensive are all from Sakurai games. There are absolutely NO stages from non Sakurai games.
Items/Assist Trophies- Knuckle Joe, Kawasaki, Nightmare Wizard, and all of the Kirby series items are all from Sakurai games.
Bosses- Marx is the only Kirby boss and is from Super Star. His design in Ultimate completely ignores any changes to his appearance in SS Ultra. An example of this is how Ultra changed the ends of Marx's wings from spearhead shapes to heart shapes. However, Ultimate ignored this alteration and made them simple spearheads again.
Spirits- While there are a few modern Kirby spirits, many of them don't even have a spirit battle and must be unceremoniously bought at a shop. Bandana Dee is also just an enhanced waddle dee in this game, which is an inexcusable sin.
Trophies- The only modern Kirby trophies were Magolor, Galacta Knight, and the Lor Starcutter. The way you unlock them can also make it very easy to miss them.