According to R28, the bumpers have to be backed by 3/4 in. (nominal) thick by 5 in. tall plywood. I started make them, then realized that I was using 1/2 in. instead. Will I be ok? Has anyone else used 1/2 in. thick backing and were fine in the past?
No, using 1/2" plywood for the bumpers is illegal - it must be 3/4" plywood. As for being OK… it’s possible you inspector won’t notice (and also entirely possible he/she will notice - you get used to feeling 3/4" thickness, and anything less will make you pull out a tape measure), and that you can live with it… but also keep in mind that many inspectors keep an eye on CD here, and it’s possible those for your regional now know to double check your backing material thickness.
Use 3/4". Your inspector may not notice (but I’d bet s/he will) but you will know that you broke the rule. Plus 3/4" plywood is pretty darn strong. I don’t feel really comfortable saying the same of 1/2".