Does RouterOS support the use of USB Ethernet adapters? If so, are there any caveats I should be aware of? Other than the obvious, like they wouldn't be connected to any integrated switch chip and therefore not capable of hardware offload.

If so, is there a list anywhere of which USB Ethernet adapters/chipsets are supported by RouterOS for hardware that has USB ports?

I ask because I'd like to use my RB5009 USB port as a management interface [rarely used, and even then low traffic volume], thereby not "wasting" one of the nice switched GigE ports.

Yes, ROS supports USB to ETH adapters and your use is exactly what they are good for. RTL8152/8153/etc adapters seems to be well supported, with exception of 8156 that may be supported soon (see viewtopic.php?t=182606). Just be aware there's plenty of Chinese clones of these chips that are super unstable...

Well that's a bit disappointing. Neither of these TP-Link USB Ethernet adapters work.

TP-Link UE300 (Advertised as Realtek RTL8153) does not show up as an interface in RouterOS
Vendor: 0x2357
Device: 0x0601

This one recognizes as ether9, but appears to have glitched out about a minute after connecting it and no longer shows as link-up, despite it being so.
TP-Link UE306 (ASIX AX88179)
Vendor: 0x0b95
Device: 0x1790

Windows 10 likes both devices just fine, without having to go to the Internet to fetch a driver for them. (I'm not holding this against RouterOS, I know it doesn't have gigabytes and gigabytes of device drivers lying around. I'm just stating this for the sake of testing. The devices themselves *appear* to be alright)

Does RouterOS support the use of USB Ethernet adapters? If so, are there any caveats I should be aware of?

This one should be officially supported (I never used it), at least on Mikrotik device with a USB A port.
https://mikrotik.com/product/woobm

An Ethernet alternative would be welcomed by sysadmins, given the risen number of SFP/SFP+/SFPXX interfaces.

woobm is a COMport-like access device ... not a regular ethernet interface

whats about not so fresh realtek RTL8156 / RTL8156B which is cheap way to add one 2.5 gbps eth port?
aliexpress is now full of them under 10 bucks

https://www.realtek.com/en/component/zo ... 0-software

If MikroTik is using the built-in r8152 driver from the 5.6.x version of the Linux kernel, then only RTL8152B (Fast Ethernet), as well as the revisions RTL8153 (original) and RTL8153B of the RTL8153 family (GbE) are supported. Supports for the RTL8156 family (2.5G) as well as the revisions RTL8153C / RTL8153D of RTL8153 only come with kernel version 5.13: https://github.com/torvalds/linux/commi ... aa79cce208

I think MikroTik should switch to using the up-to-date source code provided directly by Realtek instead, from this page https://www.realtek.com/en/component/zo ... 0-software. That version also supports RLT8153E and can be compiled for a whole range of Linux kernel versions, including 5.6.