Also, note that not every developer marks their extensions as "abandoned" (when they are no longer being maintained or updated) as most tend to leave their extensions outdated as well as not changing their Packagist information to list the package as "abandoned". This usually happens in the event that the developer no longer wishes to work on their extension, possibly want to be lazy about it, or just doesn't care about the Packagist or Flarum extension development ecosystems.
So the reality is, always look at the requirements in the composer.json of the extension and look out for the version of flarum/core it's requiring. You can also look for discussions here in the extensions section of this community if the author even created a discussion about it. At times, some extension authors solely develop extensions for their own communities and don't bother to add a proper README or bother to share the wealth with the rest of this community. All are in the norm really so this discussion can end up being very subjective and opinionated at any given time.
Be safe, install selectively, and foremost, if you are unsure about the stability of an extension (given how outdated it may be), test locally to be sure it meets your requirements and cause of concerns ?