Comments to this question were deleted as inappropriate. This surprised me, because the comments I added followed a pattern that I have been using for many years on many StackExchange sites - adding helpful information that isn't complete enough to be an answer. In my opinion they came under "Add relevant but minor or transient information to a post", which is an accepted use of comments. Have I misunderstood this? Or has policy changed?
In short, whether or not a comment is "appropriate" is no guarantee that it won't be deleted.
The key portion of the comments privileges description is:
Comments are temporary "Post-It" notes left on a question or answer.
ANY comment should be considered temporary:
When should I comment?
You should submit a comment if you want to:
Request clarification from the author;
Leave constructive criticism that guides the author in improving the post;
Add relevant but minor or transient information to a post (e.g. a link to a related question, or an alert to the author that the question has been updated).
If a comment requests clarification from the author, the comment ceases to be relevant once the clarification is provided (and the clarification should be edited into the original post, not left as a comment).
If a comment provides constructive criticism that guides the author in improving the post, the comment ceases to be relevant once the author follows that advice. The author may also decline the advice, in which case the comment may still be useful, but not necessary.
If relevant but minor or transient information is added to a post, then ideally that information should be edited into the post, instead of living in a comment.
Part of the problem is the sheer volume of comments found in SE communities. Moderators have the ability to selectively delete comments, but it is generally much easier (and sometimes less likely to result in accusations of bias) to use the functionality that deletes all comments from a given question or answer.
So even if comments have merit, it is entirely possible for them to be deleted.
However, there is another category of comments not really spelled out in the privileges section that has particular relevance on skeptics.se: comments that point out significant flaws or inaccuracies in an answer. Its not really practical to make a rule based upon this type of comments, as the flaw or inaccuracy pointed out may be subjective or disputed, but I believe we can count on moderators to exercise discretion when using the "delete all comments" functionality when important comments are present.
By taking a cursory look:
- there were 5 or 6 comments talking about the subject but not the question.
- a mod pointed this out by leaving the following comment:
Comments are meant for clarifications, not for trying to answer the question without any references.
- further comments debated the above commment by the mod and went further off topic
- mod nuked the thread 2 hours later