Is a /24 still the smallest you can advertise via BGP to ISPs? I'm hearing differently but always thought that was the rule of thumb.

Recently a VAR mentioned redundancy between sites by splitting our public /24 into two /25s and advertising from one site while advertising /24 from the other instead of using AS prepending. I specifically remember being told by ISPs they wouldn't accept smaller than /24. Is this no longer the case?