There is an official Twitter Gaming account that, unsurprisingly, talks about video games. This account recently asked users the character they would most like to see appear in Super Smash Bros. Ultimate. The official Halo account responded with a gif of Master Chief.
— Halo (@Halo) April 2, 2019
Is this confirmation of Master Chief heading to Super Smash Bros. Ultimate as DLC? Good God no. If anything, it’s probably just a social media guy having fun at the computer, trying to inspire articles exactly like this one to be written. But in this exceedingly peculiar age where Cuphead and (probably) Xbox Live are coming to Nintendo Switch — never say never!
As it stands, one of the major alleged leaks for Ultimate DLC includes a character pretty similar to Master Chief.