But its a fairly specific/topical subreddit, where a left-right score is questionably meaningful.
The History of Philosophy book series has something like this in its intro (I’m butchering it up): everyone has their biases, so the best thing we can do is to state them upfront.
A couple of ways you could answer this:
1. Where does the Reddit user community lean, taken as a whole. That shouldn't be hard to compute or guesstimate.
2. Where do the owners of Reddit lean (as reflected by the subset of their policies which can reasonably dictate political leaning)?
3. Where do the most influential Reddit user accounts and communities lean?
Computing a media bias score for social media isn't that far fetched, even if the result may be fuzzier than, say, a journal with an editorial bent.