Yeah there is no definition
https://www.mlb.com/news/check-swing-challenge-comes-to-single-a-florida-state-league
the 45 degree thing was used for various reasons, one of them being that the umps from the baselines CAN ACTUALLY SEE THE REFERENCE since they are on the lines. If the bat points straight at you, it's a swing. So if the rules shifts to that, the humans can make calls. Right now, they can't even actually tell if home plate is crossed, for example, they just guess.
It's also interesting that historically a check swing was way more liberal (see above link).
You can't use home plate because then players who are more forward in the box or drift more are at a disadvantage
Personally I like it. The 90 degree thing never really seemed like a SWING or an actual OFFERING at the pitch, to me. Like, if you hit the ball it's just a bunt...
Also, from a policy perspective, strikeouts are so high and stuff is so nasty. Pitchers don't really need those extra few whiffs, do they?