It’s recommended but not mandatory to use a different identity when sharing you server resource with any other user, because what happens is when you create a Identity it’s encrypted with your public key and when you share with a different user (in and out of your organisation), The same identity is encrypted with the other user, and the user access to identity and decrypt it the private key would be visible to them which can be saved to there system and cannot be tracked further. So when even after you revoke the access to the user from yoheim, he can access to the server with the private key he copied earlier. This is not just the case while using the platform, this is applicable in normal scenario when you are sharing a key with traditional way, you should always create a separate identity while sharing access to other users.
So this is why it’s recommended to use a different ssh key for other users or each user. In this case after revoking the access you can manually go to the server and revoke access to that identity which was shared earlier.