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..or from YubiKey for 1$ more (then their USB c version)

They sell a Yubikey 4 with USB-C adaptor which looks like what you want: https://www.yubico.com/product/yubikey-4-nano-usbc-bundle/ Looks around the same size as Yubikey 4C.

It's an authentication key, not a flash drive.

The same point still applies.

It's a comment, not a logical proof

That doesn't change the fact that you may still need to use it in a friend's computer, or at an internet café.

Could you explain why you think that? The audience for this is people that primarily use C-only devices; it was requested by customers.

I don't see the point of a dual device except to add size and expense. I'd rather just go with a dongle, as it can be reused.