This is status-bydesign.
The association bonus is purposefully removed from the calculation used to determine if you have sufficient reputation to answer a protected question.
To answer a protected question, you must earn 10 reputation specifically on this site, regardless of your reputation elsewhere on Stack Exchange.
There's already a feature request on ...
That particular question was automatically protected by the site due to the quantity of ‘answers’ the post received from new users posting things which weren't answers. Protected questions are an extreme minority of the questions on this site and it's unfortunate that it prevented a legitimate answer from you.
I have unprotected the question — go ahead and ...
Protected questions require 10 reputation to have been earned on the site itself, thus this is status-bydesign.
The 100 reputation association bonus is purposefully removed from the calculation used to determine if you have sufficient reputation to answer a protected question, and as such you have 6 reputation which is below the 10 threshold.
If you found ...
In general, questions that are "outdated" are not in need of removal, but rather in need of editing to clarify that they apply to certain versions of an operating system and not to others.
An answer that applies to Lion but not High Sierra isn't wrong or unhelpful, but does need to be edited to make clear in what contexts it is helpful.