Two quick clarifications:

Currently support forum threads auto-close after 6 months, not a year.

The auto-closure to replies is different from marking the thread as “resolved,” which impacts plugin ranking. – a plugin contributor or thread starter can mark threads as resolved and it does not stop people from replying to them or close them to comments. A thread can also be marked as resolved at any time, even after the comments are closed. This is something that perhaps is not intended, but plugin dev’s can change the resolved status at any time – it’s a dropdown separate from the comments form.

In my case, on the plugin that I had failed to subscribe to and could not reply to the 12 support threads, all were “unresolved” which hurt the plugin’s ranking. I ended up going through and marking them all as resolved, because I don’t think the plugin should be permanently lower in search because of something that is no longer an issue (me not getting notified of the threads). It might be gaming the system a bit, but since dot org doesn’t allow me to actually help the original posters, it’s the best I could do for the plugin.