So here's a starter on this topic...

I wondered when the term non-league was first in regular / common use...?
I think this guyhas a good take on the matter.

https://www.staff.city.ac.uk/r.j.gerrard/football/nl/history.html
"The first local leagues in England were formed in 1889, only one year after the Football League, but by that time the F.A. Cup (started in 1871) had already been plagued by accusations that some players had accepted money for playing. Some clubs wanted professionalism acknowledged, others wanted it forbidden. In a spirit of compromise which satisfied nobody, separate competitions (the F.A. Amateur Cup as well as amateur leagues) were set up for amateur clubs only, the other non-league competitions being semi-professional or mixed.This situation continued, not without some acrimony, until it became apparent in the 1970s that certain amateur clubs were again paying players. From 1973-74, then, there was (officially) no more division between the two halves. But when, in 1979, it was decided to unify the best non-League teams in the country in a single competition, those from the (formerly amateur) Isthmian League were overlooked and the league was composed entirely of former members of the (professional) Southern League and Northern Premier League. Thus the supremacy of the new Alliance Premier League was not initially universally accepted, especially as the F.A. Trophy (the unified competition for the best non-league teams) was more often won by Isthmian League teams than members of the APL.1981-82, however, saw the defection of two top Isthmian clubs to the APL, and from 1985 the champions of the Isthmian League were automatically entitled to promotion to the APL like the champions of the Southern League and the Northern Premier League, although promotion can be denied if facilities are adjudged not to meet the minimum standards set out by the APL.The precedent of automatic promotion from one league to another has spread throughout the non-league system, creating a structure called the 'Pyramid', whereby the three major regional leagues feed into the APL, but also accept new members from official feeder leagues. The Northern Premier League accepts the champions of the Northern Counties East League, the North West Counties League and the Northern League automatically; these leagues in turn accept new members from lower leagues. The Southern League and the Isthmian League are not obliged to take anyone from their many feeder leagues, but generally accept about 4 new members each per year.On the Cup scene, the top 450 (approx) non-League teams take part in the F.A. Cup; the top 176 also participate in the F.A. Trophy, and about 500 teams not entitled to a place in the F.A. Trophy play for the F.A. Vase."Copyright (C) Russell Gerrard and the RSSSF

I'm old enough to remember the 'Corinthian Casuals' - and the demise of the term 'amateur' game as described above. Also the strength back then of the Isthmian League...