lock-in to which vendor?

Passkeys support transfer to any vendor you want.

Can you send some documentation on how? For example, I tried googling for transferring a passkey out of popular systems and it doesn't seem possible[1][2] other than through JSON export[3] which is what some sites want to block as I understand.

[1] https://old.reddit.com/r/Bitwarden/comments/1efs5d2/how_can_...

[2] https://old.reddit.com/r/Bitwarden/comments/1di8nbz/import_p...

[3] https://news.ycombinator.com/item?id=44454106

I don't think you're going to find it. The main vendors are hostile to this workflow. I get why, any flow that can exist to export passkeys can be used by hostile actors to walk a 75-year old millionaire grandma through handing over $$$. I think however that that's just a risk we have to make the bank and brokerages accept. It's not a problem with a technical solution.

Why is it more important than protecting users? They've already added a way to share them securely.

Wasn't the discussion you responded to about how they currently can't be shared and that the vendors don't want them to be shared as it breaks their desired lock-in?

They can be shared just not insecurely. That's why they are working on a spec.

I want to transfer them to a vendor that will let me export them in plain text.

Is it really "any" vendor, or is it just the big ones? Can you transfer your Apple passkeys to KeePassXC?

I can't even find documentation on how to do the simplest transfer, from Apple iCloud Keychain to Google Chrome or vice versa.

Not yet. Apple supports export using FIDO's Credential Exchange standard. KeePassXC is working on adding that.