Just now when I tried to git push to drupal.org I got a long warning message that started with the following.
The RSA host key for git.drupal.org has changed, and the key for the corresponding IP address 126.96.36.199 is unknown.
The article Drupal.org Git Server Migration and its comments give a good explanation and discussion about how drupal.org has migrated to different servers as of early July.
The steps I took to fix the issue on my Linux Mint system were as follows, where the -R option for ssh-keygen means "remove the keys".
# ssh-keygen -R git.drupal.org
# ssh-keygen -R 188.8.131.52
# git fetch
The authenticity of host 'git.drupal.org (184.108.40.206)' can't be established.
RSA key fingerprint is 16:f5:44:6c:a1:c6:be:72:cd:98:b5:b7:7d:26:d6:14.
Are you sure you want to continue connecting (yes/no)? yes
After that, there were no further warnings in using git to access drupal.org.
Instead of git fetch probably any git command that accesses the remote repo would have the same effect, such as git pull or git clone.
Drupal.org Git Server Migration