Fox's main noticeable weakness is a lack of a consistent KO move. Giving characters strengths over their individual weaknesses generally makes them completely broken. Fox is honestly totally fine as he is. The only matchup that I genuinely think is incredibly difficult nowadays is RosaLuma, I just play Sheik for that all day. I'm not sure I agree with Top 5 but maybe like 7th or so? I don't know.
Fox kinda feels like Melee Sheik in a way in that he absolutely ***** on a lot of the lower tier characters in the game but has a hard time against the very top like EndlessRain said. I disagree with the thoughts on the ZSS MU, though. There are a lot of matchups that Fox has to basically accept that the other character has stronger options to cancel out Fox's standard aggro, damage racking game and instead of stubbornly trying to contnue that gameplan, we have to play defensively and punish.
As for nerfs, I wouldn't be at all surprised to see the jab lock go and no other changes - by that I mean we'll still be able to go from jab 2 into stuff but not just infinitely lock Lucario and stuff like that. Fox is a pretty well balanced character all in all and the jab lock is something I sincerely doubt was intentional - Nintendo seem to like patching out things that they didn't intend before they pay attention to how that affects the character's strength (see: Greninja). This patch does have the chance to move Fox up a few spaces if Rosalina, Diddy and Sheik sees nerfs. While I don't think the latter two are ridiculously annoying, they are definitely hard, y'know. Other characters being weakened is all the buff Fox needs to compensate for jab lock leaving. All that said, if they straight up remove his jab cancel, it's honestly not the end of the world but it does take out one of our KO options which is definitely annoying. If they remove jab cancel entirely, he'll still probably stay top tier with the top tier's nerfs but we'll see how other characters rise.