A good place to start might be to find out when the 'Bethlehem Carol Sheets' were first produced.. published by Biblelands
, the charity has been around for at least 150 years. These tend to be the standard carol sheets handed round at concerts, carolling and services at this time of year... Their mass production and easy availability limited the number of different carols to what would fit on the sheet (about 25 on my sheet circa 1975) and if the tune was not familiar, it wouldn't be sung.

These sheets have been available for at least 30 years, but I can't find any date for the first publication. They do usually contain a few that aren't in any hymn book ('The Holly and the Ivy' and Good King Wenceslas' for instance...) but have become part and parcel of the Christmas carolling tradition.

LTS