From a spec standpoint, acoustic modems were building over wires spec'd for bidirectional voice transmissions, i.e. symmetrical.

I'm curious about the nuances, but it seems like the last mile download/upload imbalance was created by the originating signal mode?

Download = First mile internet to ISP could be upgraded to digital, and thus grab some extra throughout by avoiding analog noise handling

Upload = First mile user to ISP was inherently analog over phone lines, and so sacrificed throughput for line noise tolerance

https://www.edn.com/an-introduction-to-the-v-90-56k-modem/

Sorta-kinda.

v.90 didn't work at all between two regular POTS-connected analog modems. In order for a v.90 connection to happen, the ISP-end of the connection needed to be a digital circuit (typically using ISDN PRI).

By being digital, the gear at the ISP-end was able to precisely and distinctly control each individual bits that would ultimately be converted to analog at a point that was physically near to the user (their local CO switch). This was what gave us asymmetric nature of "56k" v.90.

Eventually, we got good enough at learning how to handle changing line conditions and thereby twiddle the bits with a modicum of precision in the upstream direction. This allowed us to produce a standard with a bit more symmetry: v.92.

v.92 offered up to "56k" (~53k due to FCC limits) down, and 48k up.

A lot of users -- at least in the US -- never experienced v.92. It wasn't formalized until right around the turn of the century, which corresponded well with the time when xDSL, DOCSIS, and/or BRI started showing up even in fairly small not-completely-rural communities at fairly reasonable prices. The local dialup ISP market was beginning to die by then and many never bothered with upgrading their gear to support v.92 before they closed their doors for good.

(All of this wacky dial-up modem tech was both enabled and limited by digital switching in the PSTN. Speeds over dry-pair phone lines could be far higher if there wasn't a digital conversion in the middle, and avoiding that digital conversion is how DSL became possible.

Which is interesting: A DSL circuit was meant to go only across town (ish), and was always betwixt two fixed points. But a point-to-point v.90 or v.92 connection could be established to any properly-equipped machine by just dialing its phone number, and that machine could be across town or on the other side of a continent; it didn't care.)

It was 56k down, 33.6k up.