I mean I'll be honest that I don't follow Premier League stuff as much as I should (I'm more a fan of the Championship) but they're definitely being investigated: https://www.skysports.com/football/news/11679/12804623/man-city-premier-league-charges-explained-what-are-they-what-could-punishment-be-whats-the-timescale
If that investigation is over, I'd love a link. Otherwise I'm pretty sure it's ongoing. Like the Everton investigation.
Not illegal, per se, but by all indications they're going to fail FFP