1:31 left in the 4th quarter. Chiefs had just scored a TD to pull within 6. Attempting an onsides kick. Chiefs players all line up to the right side. As the Kicker goes to kick the ball, it appeared that he CLEARLY put his plant foot across the 30 yard line as he went to kick the ball? Did anyone else see this and think it should have been offsides?

Didn't see it but I wouldn't call it, the kicker is allowed to be offsides.

Yes, in NF, NCAA, and the NFL, the kicker can be beyond the free kick line/ball.

NF - rule 6-1-3
NCAA - rule 6-1-2-A
NFL - rule 6-1-5-B

Why are onside and offside pluralized for the US game?

Kinda like asking if it's RBI or RBIs.

Back to the original question - if a holder was used it would also be okay for him to cross K's line.

NFL rules state that the kicker's plant foot may be offside but his kicking foot may not be. I don't know how he could get his kicking foot offside but that is the rule.