tl;dr
The distinction between speaker and author, even in lyric poetry, isn't recent. Aristotle made the distinction around 335 BC, so it's around 2,360 years old, give or take.
Deets
In the Western tradition, the distinction between the narrator and the author goes back as far as the foundational text of literary theory, Aristotle's Poetics. Aristotle says that imitations of the world can be distinguished along three axes: the medium of representation, the objects of representation, and the manner of representation. The third axis here, the manner, is where he examines the distinction between speaker, narrator, and author.
The medium or means of representation refers to whether the artwork in question is a painting, a musical performance, a dance, or a linguistic artifact. The objects or matter of representation refers to whether the characters depicted are superior, inferior, or equivalent to a normative human reader/spectator. For instance tragedy represents superior individuals, comedy inferior ones. Finally, Aristotle has this to say about the manner or mode of representation:
For in representing the same objects by the same means it is possible to proceed either partly by narrative and partly by assuming a character other than your own—this is Homer's method—or by remaining yourself without any such change, or else to represent the characters as carrying out the whole action themselves. (1448a)
These three manners of representation have come to be known as the narrative modes, termed epic, lyric, and dramatic respectively. In each, the author or poet manipulates the relationship between the narrator who depicts and the characters being depicted.
- In the epic mode, as in Homer or in novels, the narrator speaks sometimes in the voice of a character (i.e., portrays dialogue through direct speech), and sometimes in a distinct, controlling voice that is the narrator's own, not that of a specific character.
- In the lyric mode, the narrator and the speaker are identified: it consists in being "without any such change" as the epic mode has between narrator and character.
- In the dramatic mode, as in the performance of plays, the narrator disappears. What we see is simply the characters speaking in their own voices.
So in the Poetics, there is already a distinction between narrator and character. The relationship between the two governs whether the mode is epic, lyric, or dramatic. This distinction is made toward the very beginning of the Poetics, and the rest of the treatise delves into the dramatic mode of tragedy (specifically Sophocles) and the epic mode of narrative poetry (specifically Homer). Unfortunately, Aristotle does not further elaborate on the lyric mode.
One can, however, extrapolate from Aristotle's brief comment on this mode. Here, the narrator speaks in a singular voice and does not assume any other character. Does this mean, then, that the lyric mode is self-expression? That a lyric poem represents the poet's true feelings, and that there is no distinction to be made between the poet and the speaker of the poem? Was it Wordsworth himself wandering lonely as a cloud, or Dickinson herself who could not stop for death? This is a very common point of confusion when reading lyric poetry; but the context of Aristotle's comment makes it clear that this identification of speaker with narrator is a technical choice made by the poet.
To see a lyric poem as representing the poet's actual words and thoughts is to fall into error. Because Aristotle discusses these modes in the context of imitation. Or, to use the more precise technical term, mimesis. What the poet does is imitate or mimic the real world. A lyric poet is making a narrative choice about how to mimic: not by representing clearly distinguished characters speaking in their own voices (as a dramatist does), nor by speaking alongside such characters in a distinct voice (as an epic poet or novelist does), but by collapsing the distinction between depicting narrator and depicted character. There is a unitary speaker who is both narrator and character. The lyric persona is a character like any dramatic character; the lyric voice is a controlling voice like any epic narrator. But here, the voice is the character. And it is not the voice of the poet, because the lyric poet too is creating a mimetic world, and this imitated/represented world is not the same as the real world.
Aristotle was the master of fine distinctions. In his discussion of mimetic media, for example, he carves out a space for specifically literary works, saying that medical or scientific treatises in verse are not poetry:
For if people publish medical or scientific treatises in metre the custom is to call them poets. But Homer and Empedocles have nothing in common except the metre, so that it would be proper to call the one a poet and the other not a poet but a scientist. (1447b)
He also says that prose works such as Plato's Socratic dialogues count as imaginative literature, since they are mimetic. Given his facility with such logical distinctions, it would indeed be odd if Aristotle identified the speaker/narrator of a lyric with the poet. His claim is simply that the speaker of the lyric is not distinct from the narrator, as in the epic; nor does the narrator disappear completely, as in the dramatic. Yes, the narrator is the speaker, but the narrator is not the poet. The voice being depicted is a mimesis, an imitation, not to be identified with the real world itself. The relationship of this mimesis to the real world is of course what engages the entire subsequent history of literary theory and criticism.
Reference
Aristotle. Poetics, trans. W. Hamilton Fyfe, 1927. Retrieved May 6, 2023, from perseus.org.