Equal temparament dates back at least to Bach, and he is frequently cited as an early influence on its adoption. "The Well-Tempered Clavier" (German: Das Wohltemperierte Klavier),[2] BWV 846–893, is a collection of solo keyboard music composed by Johann Sebastian Bach. He first gave the title to a book of preludes and fugues in all 24 major and minor keys, dated 1722. The composition was specifically created to demonstrate the greater versatility for key changes on an instrument constructed and tuned to equal temperament.

Helmholtz, in his "On the Sensations of Tone" ca 1870 (? IIRC) tabulates the tunings of some hundred or so old organs, some dating to a century before the book but still then in current use, and they span a "A" range of several tones. Recollection is that "A frequencies" from below 400 to above about 470 are "built into" several very old organs.

Even if a tuning fork is used, the accuracy of tuning of other notes is dependent on the "ear" of the tuner. Since harmonic intervals are about the only thing available for setting a relationship between the notes on a single instrument, it must be assumed that's what was mainly used. The accuracy with which individual instruments approached any particular "tuning," based on the measurements by Helmholtz and others since, is quite variable, and in some cases almost "laughable," so broad generalizations are of little practical validity.

Even Helmholtz documents a gradual (fairly general) upward creep in the "A" pitch. Arguments about why this happened are extensive, but one theory is that improvements in instrument construction benefited from the "brighter" sound of higher pitches.

Commercial instruments made since at least around 1900 generally were manufactured in equal temperament in order to facilitate playing in many different keys, with other instruments in varied ensembles.

Even before equal temperament was the norm for pianos, most piano tuners (and possibly organ builders?) pitched individual notes "out of tune," with an increasing "upward bias" as they moved up the scale based on the argument that "it sounds better," so there's really nothing very sacred about conforming to an arbitrary theoretical scale.

The "best" of modern electronic tuners generally include at least a dozen "different scales" to which the tuner can be set to report (or in some cases to sound) the notes of the scale. The less common "tunings" apparently have some use, by some people, but I have no idea where or by whom or for what purposes.

John