To close this out, Autodesk has determined this is a feature and not a bug, and they support their own paid storage, as I expected, with no information on third party storage that might be supported...but I can disable Activity Insights so I won't be bothered by that pesky lack of drawing history anymore...
"Our internal team has confirmed that this behavior is due to a change in how the software accesses files. Starting with the 2025 release, AutoCAD LT moved from using the OneDrive API to our internal FileProvider API to provide a more consistent experience across platforms. Unfortunately, this new API does not support retrieving drawing history for files stored on SharePoint, which is why the version history is no longer visible.
We completely understand that this is a change from the experience in 2023 and 2024, and we’re sorry for any inconvenience this may cause. At this time, this behavior is considered “as-designed,” and there are no current plans to bring back SharePoint drawing history in Activity Insights for these versions.
As a workaround, if Activity Insights is causing issues or isn’t needed, it can be disabled by setting the system variable ACTIVITYINSIGHTSSUPPORT to 0.
About your question on which storage providers are compatible with Activity Insights drawing history using the new internal FileProvider API in AutoCAD LT 2025 and 2026, there isn’t a specific list available at the moment. However, since Autodesk has shifted to using this internal API, it seems that Autodesk’s own cloud storage options, like Autodesk Docs and Autodesk Construction Cloud, are likely the ones fully supported.
If you’re looking for more detailed information about other third-party storage providers, I’d recommend reaching out to Autodesk Support or keeping an eye on Autodesk’s official updates, as they might share more details over time."