Looks like you're correct, I apologize. I don't know why I remembered it as an optional rule, maybe because we have it as a house rule.
The standard for backgrounds are two pre-defined skills though, even if we can customize them in PnP. I suspect Larian didn't implement it, because they somehow assumed everybody and their dog would take perception or persuasion...
You renember it as an optional rule because it is an optional rule. It says 'customize' a background. To customise something you need to change it.bor make up your own. Which means you need DM approval to do it. If not they would just say 'pick 2 skills, this many languages, this many tools for each class and not even bother with listing backgrounds to choose from.
Its not used overall because custlmizing to such a degree is generally only done by metagamers or people who otherwise inconveince the other people at the table. People who just MUST have a specific skill for example but dont get acces to it via their race and class alone. And dont want to spend a feat to get it either.
Its a fine option for a video game though. Dont see the harm in it. But mind you also that the custom backgrounds dont get any background features. At all. And some are VERY usefull.
