I'm fine with the rule "If you hit a player who has one stock with an upsmash, and that player is at 300% or more, and your opponent is not playing Snake (grenades), and you are not playing Snake, and the stage you are playing on has no hazards, and neither of you are playing Peach, and neither of you are playing Diddy (barrels), and no waddle dees are in play, you may pause the game as many times as you want, for any reason."
That would allow pausing only in cases where it really, really could not change the outcome of the match.
What you're proposing isn't nearly as discrete though. It's people making a judgment call on what it takes to "definitely" kill someone. Snake uptilts a Meta Knight at 160% and pauses, the judge is all like "oh yeah bro you can't DI that", the MK saying "he totally staled it enough he had 4 uptilts in the queue", the Snake is saying "lies and even then it doesn't stale enough", "you wouldn't know you only ever play with MKs that don't buffer the UpAir right", rahrahrahrahrah.
Whenever possible, you design rulesets so that they don't have rules that require subjective assessments. Either the game automatically tallies what the rule impacts for you (like LGL), or you make the judgment call so easy that even in borderline cases the call is clear (whether a basketball went through a hoop or passed by it).
Subjective rules only occur out of absolute necessity. Figure skaters have them because without it, they can't compete at all. Basketball has them because without them, unregulated contact would turn the game into an outright brawl.
Allowing people to pause at the end of matches to save minute amounts of time or to zoom in on their opponent's character is not even remotely a necessity, so it's a bad idea to introduce a subjective rule.