Beginning November 13th, 2020, we will no longer accept account passwords when authenticating with the GitHub REST API. In the future, we will similarly no longer accept account passwords when authenticating Git operations.
To prevent irrelevant keys from being offered, you have to explicitly specify this in every host entry in the ~/.ssh/config (on the client machine) file by adding IdentitiesOnly like so:
Host www.somehost.com
IdentityFile ~/.ssh/key_for_somehost_rsa