"Program Files (x86)" is used by some software for its own application related files, so I would not recommend using that for user data also. I generally make a completely separate new top level folder (such as, "C:\data") that is then not exclusive to any particular computer user. Any valid computer login can access it equally well. Thus if some user profile problem ever dictates making a fresh new user login, your data will not then find itself locked by the access permissions of a former login.
But you may want the data to live in a computer-user dedicated area (such that any other login accounts on the same computer could not access that). By initial default, with OneDrive not yet set up, a given computer account's own standard Documents and Pictures and Desktop and such, live directly within C:\users\[username] folder. Once OneDrive is told to start syncing user files, these user folders automatically change their locations, moving inside C:\users\[username]\OneDrive instead (this behind-the-scenes move is not evident to the user who seems to continue accessing the same files on their "Desktop", or within "Pictures", or whatever. But LrC will know this has changed since it operates on absolute file system paths. For the same reason, OneDrive causes something on the Desktop say, to appear permanently present - which in fact is sometimes not present locally, in which case it would need to be fetched down from cloud to open. That's generally more or less invisible to the user. But not when it comes to LrC: which is not so easily fooled. LrC does want the file to be genuinely present, and not just virtually so.
Anything nested within that C:\users\[username]\OneDrive folder is automatically involved in cloud sync AFAICT. But for example a new folder C:\users\[username]\data would avoid OneDrive involvement.
All this does not prevent one from still using the Pictures folder for other purposes than LrClassic. It is is relatively benign if your imported image files are here, and synced to OneDrive, provided set so a local copy is being always left in place too. And so far as JPG exports from LrC, I find Pictures the natural and convenient home for those. I do like those being synced to cloud; and these exported files being no concern of the Catalog's, no difficulty arises.
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