Sorry for what may be a naive question, but what are the retention schedule rules around a company on-prem O365 wiki page?
Not a naive question at all. However, it totally depends on the content of that page(s). Retention isn’t based on the nature of the container (ex. a wiki page), it is based on the content in the container. So, if the content represents something that should be retained for any sort of compliance reason, you would keep it for the period of time that the compliance dictates. If there is no compliance driver, then you just have business retention and that is up to whomever owns/is responsible for, that content in your organization.
I think there has to be a discussion on acceptable use on the wiki itself. For instance are you simply placing reference information that is housed in another location or is this wiki a stand alone information store. Determining how you deal with these questions will also answer those around retention. For instance, where I work, anything on the intranet including internal wiki pages are considered reference only and any documentation or information stored therein is required to be captured in another location as the official document/data of record that lifecycle governance is applied to.