That is weird. And if you change the drop down to view the 2014 documentation, it doesn't specify a version. But it's there. That happens with a lot of the SQL Server online documentation I've seen - where it erroneously states "Starting with SQL Servver 2016". So identifying versions when something was introduced isn't as easy as it used to be. It's inconsistent.
This documentation about ColumnProperty is what the question is about though and it says starting with SQL Server 2008 with IsColumnSet and in the lists of values. It specifies GeneratedAlwaysType as starting with 2016: