I did some digging, and it seems that the rules for FBA are highly variable for different vendors. Larger vendors, or vendors who have been part of the FBA program for a long time, seem to still be able to comingle, but there's a lot of confusion about it. No one quite seems sure if it's something that only old vendors get (grand-fathered in from back when it was a standard part of FBA) or something that large/trustworthy vendors get (offered on a case-by-case basis). It might even be both.

As best as I can tell, it seems that commingling is happening today, and may account for the bulk of FBA transactions (due to a few very large/old vendors still doing it), but the overwhelming majority of FBA vendors cannot commingle, many have no idea that commingling even exists, and if you signed up today you would not be able to commingle.