Paths do remain closed, but: there are some issues with subdividing the closing segment (when subdividing by number it seems to generate one segment less on the closing segment), and it likely doesn't work with multiple subpaths.
My recommendation would be to flag each line segment as closing or non-closing. There is even a chance that this wouldn't actually break any extensions (at least not more than they already are).
Paths do remain closed, but: there are some issues with subdividing the closing segment (when subdividing by number it seems to generate one segment less on the closing segment), and it likely doesn't work with multiple subpaths.
My recommendation would be to flag each line segment as closing or non-closing. There is even a chance that this wouldn't actually break any extensions (at least not more than they already are).