I have a the old XTR crank with 46-34-24T but need to replace the FD in a new frame. However, I don't think any of the new FD's in Shimano line up will accommodate a max 46T.

I have a '98 XT FD with a 46T running fine. Can't speak for newer models but the problem is more a matter of the difference in size between the smallest and largest rings rather than the size of the outer ring. Just move the FD up higher until the outer side of the cage (just) clears the highest teeth throughout its range of movement.

Although FDs are designed specifically for different diameter rings (cage profile matches ring diameter) e.g. road vs MTB, they are pretty tolerant of different sizes. Plenty of weight weenies run road FDs on MTBs.

I have an older xtr crankset with those standard rings and had like a '98 xt derailleur thats worked well...tilting the rear tip of the derailler in a bit will help shifting to the smaller ring...but too much narrows the effective width the chain can move before scrubbing (I have grip shift so it's easy to compensate for).