Your question is a good one. A simple answer will be inadequate. Here goes!
In Cornwall in about 1800 the travelling folksinger and storyteller Anthony James sang and accompanied himself on the fiddle. But he was the exception. Whilst there were many village musicians who played fiddles, fifes etc, at that level in society there were not then very many 'accompanying' type instruments. At a vernacular level such instruments seem to be a 19th century phenomenon.
The practice of accompaniment in art music certainly existed in Elizabethan times and earlier. But this was principally high status music and was probably less true of vernacular music, just for economic reasons.
There is evidence to suggest instrumental 'doubling' of melodies in 12th and 13th century church music, some of which also had secular lyrics.
In Brythonic areas, principally Wales, the skill of accompanying was highly valued. From the 15th century (and probably a lot earlier) harpers and crouthers accompanied poetry and song.
Subjectively I feel that now we are subject to quite a lot of 'folk fashionable' perceptions based on the experience of the folk revival and, quite rightly, on personal preference.
Hmm. Don't know if that helps or not!
All the best
Crowdercref