So, taking "personal" to mean specifically that they belong to an individual as opposed to a service account, yes, that definitely has happened in real security incidents with big consequences.
There's a 2008 Fedora incident of this sort, a Fedora Administrator's private key was "stolen" by bad guys and used to upload replacement packages which is well documented e.g. https://lwn.net/Articles/326170/.
I think we should assume that this has also happened plenty of times to organisations which have a default posture of not telling you about incidents at all unless required by law.
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So, taking "personal" to mean specifically that they belong to an individual as opposed to a service account, yes, that definitely has happened in real security incidents with big consequences.
There's a 2008 Fedora incident of this sort, a Fedora Administrator's private key was "stolen" by bad guys and used to upload replacement packages which is well documented e.g. https://lwn.net/Articles/326170/.
I think we should assume that this has also happened plenty of times to organisations which have a default posture of not telling you about incidents at all unless required by law.