Is such use of Github in compliance with their terms of service?
I've been thinking about using such public git providers to store small amounts of data, possibly encrypted. For example important documents that I don't want to lose. It seems that it'd be ok as long as you manually create the account.
Not really although you probably need to use quite a bit of traffic before it becomes a problem. Your example of personal use only seems fine.
There was a post here a while ago (that I cannot find anymore) about the devs of a package manager of sorts who were asked kindly by Github to do something about their excessive data usage. So it's probably not a good idea to build a company on it.
I've been thinking about using such public git providers to store small amounts of data, possibly encrypted. For example important documents that I don't want to lose. It seems that it'd be ok as long as you manually create the account.