I'm not sure what UX you are talking about, the majority of the websites supporting u2f/passkey have UX to manage your u2f keys/passkeys. (the only exception I can think of is early Twitter when it first implemented u2f, and at that point it only allow you to add a single u2f key, but even Twitter fixed that later and supports multiple keys now).
And (this is probably not emphasized enough) you really should never only use a single u2f key/passkey for a website, that's the recipe to get you locked out when you can't find your u2f key/get locked out of the provider of your passkey. I have at least 2 yubikeys on my keychain all the time (one for usb-a and one for usb-c), plus one for each of my computers, and passkeys from 1password, google, etc.. And whenever I add u2f keys/passkeys to a website I add all/most of them.
And (this is probably not emphasized enough) you really should never only use a single u2f key/passkey for a website, that's the recipe to get you locked out when you can't find your u2f key/get locked out of the provider of your passkey. I have at least 2 yubikeys on my keychain all the time (one for usb-a and one for usb-c), plus one for each of my computers, and passkeys from 1password, google, etc.. And whenever I add u2f keys/passkeys to a website I add all/most of them.