Is it on topic to ask about the specifics of established homebrew content (such as the house rules used by Matt Mercer on Critical Role) or where such things can be found?
Questions about homebrew content can be on-topic
Homebrew questions aren't categorically forbidden. Taking the Matt Mercer/Critical Role homebrew / 3rd party content you mentioned, there are a handful of decently received questions about aspects of it (example). There are also various other questions about homebrew, which can mostly be found at the homebrew tag, or the related house-rules tag.
The specific types of questions you mention have some potential to be problematic for other reasons. When asking about "the specifics" of homebrew, there's a chance of treading too far into either "too broad" or "opinion-based" or sometimes "unclear". For example, this question approximately of the form "My DM said this item exists what does it do?" can't be usefully answered because we aren't capable of reading a DM's mind like that. Critical Role content is less likely to fall into this category, especially if you can link and/or quote a source for its rules text. "Where can I find X?" is essentially covered in a prior meta question, and the conclusion seems to be "probably off-topic" but you can ask in chat.
With regards to whether being "established" has an effect, it mostly means that you're likely to get better answers due to greater familiarity. It shouldn't have a substantial impact on whether your question is on-topic or not.