There is a beta Pages Mail and Data Merge solution by a third-party developer (Sal Soghoian) for macOS Mojave 10.14. It is an AppleScript/Objective-C application that permits spreadsheet data to be inserted into pre-defined Placeholder Text fields in a Pages document.
Previously, Sal had released a Pages v5.5 Mail and Data Merge application for Pages v5.5 and later, that worked into 2017, and then stopped working. This beta is the update for that older application. As a beta, you cannot be assured that it will work perfectly until it is no longer classified "beta."
On the above beta location, there is also an AppleScript that permits Pages Mail Merge from a Contacts Group.
These are just direct links to the pertinent content from Alan's Jan. 2015 link to MacWorld.