The last appveyor build failed to publish to github (https://ci.appveyor.com/project/nickgammon/mushclient/builds/42883605). That suggests to me that
https://github.com/nickgammon/mushclient/blob/master/appveyor.yml#L9
and
https://github.com/nickgammon/mushclient/blob/master/appveyor.yml#L26
are both still using the old API token format that Github doesn't accept anymore.
You need to generate a new Github API token at https://github.com/settings/tokens with at least public_repo access, paste it into https://ci.appveyor.com/tools/encrypt and then replace the two values above with the encrypted output.
https://github.com/nickgammon/mushclient/blob/master/appveyor.yml#L9
and
https://github.com/nickgammon/mushclient/blob/master/appveyor.yml#L26
are both still using the old API token format that Github doesn't accept anymore.
You need to generate a new Github API token at https://github.com/settings/tokens with at least public_repo access, paste it into https://ci.appveyor.com/tools/encrypt and then replace the two values above with the encrypted output.