For example, this topic. It was necessary to add additional information for future readers of the thread, but there was no reason for my post to elevate the topic to the top of current discussions.
Additionally, it could be configured such that new user’s posts do not elevate a topic by default (until they earn a certain trust level), if a particular forum admin felt it was necessary for their community.
Posts that are old and/or only apply to one user might fall under this category as well. (See this post.)
My thought was a simple checkbox while composing a reply. I’m interested to see if anyone else has different ideas.
This may be plugin territory rather than core functionality.