The option to share a list still appears in my iPhone 13 Pro with iOS 15.2. It appears that the ability does not exist for sharing a Smart List. I just created two new lists on my iPhone. One a standard list and the other a Smart List. I believe the reason you cannot share a Smart List is because it only exists on your iPhone based on the face it is created by filtering existing lists to be created. So, in essence, it is a compilation of multiple lists on the iPhone, so it would almost be the same as sharing all of your lists at once without the filter. So, while a regular list will share, a Smart List will not because of its construction.