A Floating Island was introduced in the first episode of Blast To the Past. I like to think that Ben Hurst was hinting at something that could have related to Sonic 3 in some way.
https://sonic.fandom.com/wiki/Floating_Island_(SatAM)
Maybe one of the SEGA corporations red-flagged Hurst's idea and maybe that's why Knuckles never showed up in SatAM.
As for Metal Sonic, SatAM did bring Uncle Chuck into the series. Metal Sonic wasn't dwelt on much since Sir Charles was a temporary antagonist until he took hold of two Power Rings within two different episodes, and eventually defected to The Freedom Fighters cause.