In the old Exhibition Gallery there was special configuration which prevented threads expiring because these were considered to be generally items of long or longer term interest. It appears, however, that this configuration has got lost with the category rename to Showcase. Further, it seems that new posts do not refresh the expiry date.
It is not related to the category restructuring. The regression occurred a few months ago when I did a comprehensive survey for categories in which the auto-close setting was unintentionally not enabled:
You are right. I made a mistake when I enabled the auto-close setting for the category. This "Don't close until the last post in the topic is at least this old." setting is disabled by default, so we must toggle the setting when we enable auto-close for a category. However, because auto-close had already been enabled for this category previously, that setting had already been toggled, so when I toggled it again without noticing that fact, it changed it back to the disabled state.
I have removed the auto-close timer from that and all the other affected topics.