Currently I don’t believe there’s a way to make a post sticky within a topic. I think when this question has cropped up before one suggestion was to make the second post in the topic a placeholder for such a summary. This would require some forethought though to make sure a suitable one was created at the time.
It would be possible to manually move all the posts in an existing topic into a temporary topic, create a new placeholder post, and then move the posts back again but I think it might be a faff.
There are some new AI summary tools that appear at the top of topics which may prove useful?
I’ve managed to find a very old feature request for it, but it didn’t gain much traction:
I think I have seen another one somewhere. 