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Hi, @Robin Horn NZ ,
First, for SharePoint Online, you understanding is right, versioning is enabled by default for SharePoint Document Libraries since an update started in 2018. For libraries created in February 2020 and May 2020, they shall be enabled on versioning from the very beginning. And after this update, end users shall not be able to disable it via UI. There is no option to disable it in list settings.
So for now, if we want to disable versioning, either we need to use PowerShell to set it directly. This approach will needs site owner or site admin permission. Or it can be configured via developer API. Audit logs will not provide trace of list settings change, so I will suggest you to direct check all the owner or admin users and ensure there will not be further changes on versioning settings.
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