I've read that theory, too, but ဂ is written with just one stroke and if you look at palm-leaf manuscripts in general https://en.wikipedia.org/wiki/Palm-leaf_manuscript there are plenty of angled shapes. (Ironically, the example Wikipedia uses for Burmese has a more square style instead of the now-dominant curves.) So I wonder whether it's all just a post-hoc rationalization for historical happenstance. It would be interesting to know whether e.g. Sundanese ᮕ is similarly written with two strokes.
replySo many of these scripts basically derive from Brahmi which was famously rock-cut (Ashoka monuments). That type of medium is well suited to angles. Major scripts including Tamil were also enscribed on to metal .. this image shows an evolution directly away from angular toward rounded. https://www.instagram.com/p/CpB1ygEPwJt/
replyPresumably this reflects a shift from courtly scribe closed-door literacy toward more popular literacy specifically including temple-trained monks who presumably could not have afforded either stonemasons or metal plates for general use.