a bit before my time, but on this side of the pond a group called QRP ARCI was among the pioneers. funny thing they started with QRP being 100 watts or less (was this input or output power okay trivia) but the leadership pushed the matter of limiting QRP to 5 watts or less output power. was this because it was a then perceived transistor limit?
now the matter of QRP SSB is a bit controversial -- some contests require 5 watts or less but there is some convention for allowing 10 watts. yes when the ions are strong SSB can do QRP - I have had some surprising contacts with my uBitx. and I remember a QSO with an OM who was portable near the sea when I was using my FT817 into my 2 element yagi likely on 20m.
okay one can cite these limits as convention, I think the OM referencing the Bible was tongue-in-cheek ... but in an older English translation I think the word solder does appear there.
for WSPR my view is the barefoot U3S is 'real qrp' for that mode - as 200 mW crosses oceans on 7 and 10 MHz rather easily from my verticals. but I confess I need to QRO on the lower bands.
enjoy your radiations all