> No! A set of bits exists on disk. A piece of software (git) interprets those bits as a branch

I call what you describe there an abstraction. So, feels like I'm going full circle here to my original comment.

My (possibly incorrect or incomplete) understanding is that abstraction is when some complex underlying mechanism is presented in a manner that serves a specific purpose. In essence the difference between an abstraction and a social construct is that an abstraction deals with simplification of ideas and presenting them in a manner that makes it easy to interact with them.

While social constructs are about shared understanding of ideas regardless of whether these are reified in any manner or whether there is are ways to interact and engage with them.