Not sure.

140 countries have signed onto the push to cap passport validity to five years. Most of those that have reduced their validity period down to no more the five years don't allow for adding pages and those that did have all stopped it in so far as I've not seen them allow it anymore. The US has maintained a ten year general validity period for ordinary adult passports and doesn't seem as likely to kill that off than it is to kill of the additional page inserts -- but neither is scheduled for elimination even as the U.S. is an advocate against other countries allowing for page inserts or allowing for passport validity periods to be greater than 5 years. It's rather amusing when what's good for the goose is not considered good for the gander.

It used to be more common to see multiple passport books bound together than to see additional passport page inserts in non-US passports. It probably still is, for the latter seems to have largely dropped off the radar.