I know 56k were the fastest on the sticker, but going forward, there does not seem much point in a "56k or nothing" approach to obtaining a modem to work with in future, whether for retro or crap hits the fan. The copper is rotting, it's not getting replaced, it's getting superseded. Even "landlines" many places are implemented as VoIP over fibre for new installs. Even if you've got copper, and your pal down in Florida has copper, if you call him long distance you're gonna be going over a VoIP trunk in between now. So that leaves 56k useful for phone lines where you know you've got copper, to call an ISP class 56k modem on the same exchange... got any very local ISPs or BBSes that have 56k modems of the type that are hardwired in? 56k P2P, consumer to consumer was never a thing. 33.6 is the fastest you ever got. Okay, I know I know, if you get a 56k it should be backwards compatible with every other standard. 33.6 may be do-able copper to copper on the same or close by exchanges. So 33.6k is the most modem you're ever likely to make use of, and line quality determines whether it can hit 33.6 or not, which is mostly going to be at shorter range than longer range... which means if you're going out of your area... and still managing to stay on copper, 28.8 is probably more like what you'd connect at, and the difference in experience between that and 33.6 is more like between 10 in the morning and noon rather than night and day.....
But you wanna connect to these cool retro BBSes that people are still or just started running, across the current telephone network that has VoIP trunks in it for longer distance. Now they might be fairly high quality VoIP trunks but in general they only have to meet the standards for Class II fax transmissions so 14.4k uncompressed line speed. And this also might be where you are if you've got a land line, even from the historic monopoly "Telephone company" and it's VoIP either at a box in your house or converted on the pole. I have heard reports that 14.4k is theoretical and the most you really rely on is 9600 🙁
Anyhoo... everything should be backward compatible to lower standards, but today now, a 56k doesn't get you much more than a 33k does, and that only in ideal non-digital line conditions, which more likely favor a 28.8, but with anything digital in the way it doesn't even beat a 14.4k. It's your money to waste on chasing the lump of hardware with the best numbers on the sticker, but if you really want to use it, don't pass up decent 14.4 to 33.6 modems for peanuts.
Also a note that line connection speed of the modem and serial port speed are not connected... you should be able to establish a 33.6k connection with a 38,400bps 16450 or 8250 uart... you just won't get much advantage from compression, or keep the pipe filled as it were... for P2P gaming, you might even want compression completely off to remove some latency... however, you can still get 4:1 compression on a 14.4 connection, so you might want to ensure you have the 16550A to use 57,600bps for best throughput. (Mostly applicable to external)
I highly doubt the state of the copper will improve after a crap hits the fan scenario, and the VoIP in between could well be dead, in which case looking to run mesh networks on WRT54Gs or other hackable routers seems much more viable.
Unicorn herding operations are proceeding, but all the totes of hens teeth and barrels of rocking horse poop give them plenty of hiding spots.