The 1954 definition was an improvement on what came before it in a number of ways ... it had an international focus, it was more inclined to allow in literary sources or was more aware that there had not been a purely oral culture for a very long time.

I first read about it in Lloyd's Folk Song in England

Reading and rereading through articles written by Bert Lloyd I found his 1979 definition of 'Folk Song' in a collection called Folk Music in School (p10)

"I would suggest that nowadays by 'folk' we understand groups of
people united by shared experience and common attitudes, skills, interests and aims.
These shared attributes become elaborated, sanctioned, stabilised by
the group over a period of time. Any such group, with communally
shaped cultural traits arising 'from below' and fashioned by
'insiders', might be a suitable subject for folklore studies. Some of
these groups may be rich in oral folklore (anecdotes, speechways,
etc.) but deficient in songs; others may be specially notable for
superstitions and customs. Perhaps for English society the most
clearly defined of such groups are those attached to various basic
industries: for example, miners with their special attitudes, customs,
lore and language, song culture and such. But it will be seen that my
suggestion does not rule out the possibility of regarding hitherto
unexplored fields, such as the realms of students, actors, bank
clerks, paratroopers, hospital nurses, as suitable territory for the
folklorist to survey.

The present-day folklorist, who views the problem in its social
entirety, and extends his researches into the process by which
traditional folklore becomes adapted to the conditions of modern
industrial life, has to consider the classic 'peasant' traditions as
being but a part - the lower limit, if you like - of a process by
which folklore becomes an urban popular affair. Indeed, as far as song
is concerned, that is the present stage of folklore development:
nowadays there is far greater use of the folk-song repertory and of
folkloric forms of creation in our industrial towns than in the
countryside."

Seems to me to be pretty broad but not so broad as to become meaningless and to take into account an understanding of the importance of industrial folk song or folk music in an idustrialised era.