Yes bleach can damage porcelain toilets especially if you use too much bleach or use bleach too often to clean your toilet.
However occasionally using some bleach to clean your toilet bowl should do no harm to the porcelain toilet.
Overtime though if bleach is used too much it can eat away at the porcelain surface on the inside of the toilet bowl.
It's best to use toilet bowl cleaner or some soap and water and even some baking soda to clean your toilet bowl.
However using bleach in your toilet bowl occasionally is okay and should do no harm or damage to your toilet bowl.
I sometimes will use bleach in my toilet or pour a bucket of water and bleach down the toilet after mopping my floor and it never has harmed it.
I also sometime use bleach to disinfect my porcelain bathroom sink as well and it's never been an issue.