I think Freebird is a pre-Internet meme, like "where's the beef" was. When someone would take song requests, shouting out Freebird was asking for a song known for versions with 20 minute dueling guitar solos. Basically a ridiculous and over the top request.

It is a good song since it's still popular today in a serious sense many times, which is what has kept it still somewhat relevant in pop culture.

Source: am somewhat old.