Just for reference. There is no obvious link on the GUI to display again the muted topics on a category to unmute them.
As @ezekielf probably has noticed, the default action in a category is to show “latest” posts. Also indicated by a big red button. As muted things do not show up in “latest”, they are not visible there. Similar reason that the muted categories are not visible in the global “latest” section.
To view them again, I add this to the URL:
?state=muted
to view the muted topics, it would be:
https://community.openstreetmap.org/c/communities/th/53/l/latest?state=muted