Before I start: I thought about the matter when reading this answer to the How is the community doing?  post, which claims a larger percentage of negative reactions towards homebrew questions compared to other questions, and also urges more open-mindedness towards homebrew questions. The latter could potentially be relevant when answering this question, while the former will be of relevance down below.
To provide a short summary, "is this balanced" questions (RPG SE search query) usually present a homebrew "thing" and ask the community about their assessment of its balancing.
While I personally like these questions, both because it's refreshing to see other people's creative content (so-to-speak) and because answering them allows me to participate in this creative process, I don't feel like they fit in with the general objective and evidence-supported answer style that is commonly encouraged (or enforced) on the site. Sure, we have the knowledge and experience on the site to evaluate the balancing aspects of homebrew - but so do /r/dndnext or many other forums.
Now, usually, homebrew questions should be supported by experience instead of simply being reformulated brainstorming results. But how could you possibly provide an experience-based answer if the only person who has ever been in contact with the respective homebrew so far is its creator, who also asked the question? I highly doubt that anyone is ever going to take some potentially unbalanced homebrew (or even their own, rebalanced version) and actually playtest it, just to provide an answer to an RPG SE question. Yes, it might happen on occasion, but I don't believe that those very rare (if not legendary, to use 5e game terms) cases should be relevant for answers to this question.
Another argument against such questions is that they're not really going to help future readers - which doesn't automatically disqualify a question, but hugely limits is usefulness (after all, providing signposts is one of the main upsides of duplicates).
Asking homebrew questions such as Does not having flight nerf a warlock's familiar? could be useful for future readers, and not just for someone who wants to restrict familiars with the ability to flight, but also someone who's trying to figure out what to actually do with a familiar (RP aside).
Questions such as Is this homebrew elemental spear balanced?, however, are highly unlikely to help future readers, even if someone else needs to balance an almost identical homebrew item of their own (or would you do an extensive, unlikely-to-yield-useful-results google search to see if someone already made a similar item, before you simply asking right away if it's balanced?). The only thing those questions are good for - aside from solving OP's issues, of course - is giving someone reading the question ideas for their own homebrew items.
While researching existing questions before posting this, I also stumbled upon these questions: Are homebrew evaluation questions OK? and How to deal with feedback questions about house-rules?.
Both of the linked question's answers would classify most "is this balanced" questions as unfit for the site. Still, many of the questions that are listed by the search query mentioned above have neither been closed as "primarily opinion-based" (or for similar reasons), nor have they have received notable numbers of downvotes (questions that show a lack of research effort aside).
Therefore, I believe we should establish a policy of sorts on how to treat the described questions currently floating around somewhere between the balance- and homebrew-tag. In the case that they are considered on-topic, then either a custom tag or changes in the description of the balance tag would probably appropriate - I personally can't really tell what the 7-year-old phrasing
Questions about perceived inequities between characters derived from differences in capability.
actually tries to tell me - but it's not what I would expect from the "balance" tag.