In typical 19th-century harmony, was a Plagal Cadence always used with an Authentic Cadence preceding it? To put it another way, was it typically ever used out of its "Amen" context?
The nineteenth century is very broad; it had some of Haydn, Wagner, and Debussy!
But certainly in the first half of that century, the authentic cadence (V to I, both in root position) was supreme and treated as the "true" "form-defining" cadence. Plagal cadences (or "plagal motions," as some call them, to hint that they're not true cadences per se) typically function as what we call "post-cadential," meaning they occur after the "real" cadence of V–I that happened somewhere earlier in the piece.
But starting somewhere in the middle of that century—maybe two thirds of the way through?—we start to see composers using that plagal motion more and more often, and occasionally those moments are even used to end entire sections where there don't seem to be any authentic cadences in the immediate vicinity.
The conclusion of Wagner's Tristan und Isolde (completed 1859), for instance, ends not with a V–I motion, but with a series of plagal moves (as does his Ring cycle, completed in 1874). One can certainly argue that there are prior V–I motions that may be the "true" cadences, but nevertheless this shows the slow change in hierarchy of these cadential types.
This further evolved into practices like we see today: where IV to I can be understood as a proper cadence (as can any other number of chord progressions), depending on the style of music.
JS Bach seemed to do everything under the Sun in his 371 Harmonized Chorales. This opening phrase caught my eyes several months ago, because it appears to me to end with a plagal cadence...
If phrases end with cadences, and the fermatas in chorales mark those phrases, is there any reason to not regard that first passage a phrase with an ending cadence?
If it is a cadence, and the harmony is clearly
IV I, is there a reason to not regard it as a plagal cadence?
Of course I'm not directing those questions to you, they are rhetorical. I think this must be regarded as a plagal cadence.
Some might say the passage is an elaboration of
I IV I. But, IMO that's just a rationalization to not call it a plagal cadence. The sequential
[I V] ... [IV I] and secondary dominant to
IV are too much to gloss over.
Stylistically the chorales touch on older harmony and aren't the periodic and sonata structures of the later classical style which focused on tonic/dominant harmony. It's in the classical style that plagal cadences aren't considered proper cadences. JS Bach was famously a transitional composer between the old and new classical style. So this example represents the older style with different sensibilities about harmony and cadences.
Personally, I think the so-called "Amen" plagal cadences aren't good examples of cadences. They are codas. You can probably find better examples of real plagal cadences, ending real phrases. But you probably won't find many from the classical style.