We don't use md5 for private/public keys because md5 is a hashing algorithm, unrelated completely to encryption. Also, what are your reasons to believe that md5 has been used there?
In certificates, md5 - and sha1 - was used quite some time after it was known to be weak, I suspect OP was thinking of that.
This article seems to give a good summary what happened with sha1, mentions md5 in passing and links the related chromium issue: https://konklone.com/post/why-google-is-hurrying-the-web-to-...