When using a YubiKey with a mobile device over NFC (tapping the key to the device), you will encounter a pop-up that links to this URL (https://demo.yubico.com/yk). This behavior is intended and referred to as the NDEF / OTP pop-up. If you'd like to disable this functionality, you can do that by turning off the OTP over NFC functionality from the YubiKey Manager, under its Interfaces tab.
Note: This disables the OTP over NFC functionality, but will still work via direct plugin. You can re-enable this functionality at any time.
*NDEF OTP is used in certain applications, for more information on this please see the following article.
This can also be turned off in Yubico Authenticator for iOS. To do this, tap the three dots at the top of the screen > tap Configuration > tap Toggle One-Time Password > turn off One-Time Password.