#1. Wrestlers can't be eliminated by Superstars who aren't part of the Royal Rumble match
As part of last night's Royal Rumble match, Omos made his way to ringside and eliminated both Big E and Rey Mysterio in order to help AJ Styles.
While outside interference has been a major part of continuing feuds in the past, there was a time when WWE stars couldn't be eliminated from the match by wrestlers who weren't involved in it.
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A prime example of this was at the 1996 Royal Rumble when Shawn Michaels eliminated Vader and the Superstar was so upset about it that he got back in the ring and bench pressed Michaels over the top rope.
It was noted that because Vader was no longer part of the match, he couldn't eliminate Shawn Michaels from the match. Of course, this was WWE covering for the error because HBK then went on to win the Royal Rumble that year.
As noted, this form of elimination has continued over the past few years and is now commonplace in the Royal Rumble match.