Robert wrote:
>>ordinary people's burials became a
standard, or expected, thing. Was it during the 1700s or earlier?<<

coffins for ordinary folk become common with the rise of cemeteries (no pits)
outside of town and individual memorialization. Middle to late 1700s. Before
that the shroud was the common method.

Edward Bell's fine reference volume will get you some good specific sources.
Aries,
Benes etc.

Best,

Gene Prince