.XLS is alive and well. Sadly, of course, but it's a fact of life. If only because some small percentage of folks still use it daily and because many software packages that export to Excel do so to an .XLS file, not an .XLSX file.
The following may or may not be of aid, but looks interesting, especially beginning on page 88 (though I was skimming, so it may not be as useful as it looked like it could be):
https://interoperability.blob.core.windows.net/files/MS-OVBA/%5bMS-OVBA%5d.pdf
This is a Microsoft document on a Microsoft site, so it seems likely its link will not break any time soon. That said though...
I found the link on this helpful set of folks' site (credit where credit is due):
https://software-solutions-online.com/vba-project-bin-and-protection/?nowprocket=1
It is really more about .XLSX, I believe, though the document above is suggested as a Microsoft effort to provide documentation for versions prior to the open specifications approach.
Ordinarily I'd suggest opening the file package as a .ZIP package and scour it for mention, then experiment with how to remove the information directly, safely, in order to be able to move forward until a better solution is found. But I don't know that any, much less many, of the older versions (yeah, there were LOTS of .XLS versions, not just one or two), have an openable/editable package. I didn't do that in those days and have never had reason to do so now that I sometimes do do it for .XLSX packages. So I have no experience there.
Lastly, in the spirit of moving forward in some useful way, no matter if it is a distasteful one that is not helpful to the question itself, one CAN of course, open a new .XLS file and copy over all pages, assuming one has knowledge of/access to them all. Should be clean of any VBA material or references. Of course, it'd be diagnostic if it were not clean of such as that would indicate references traveling in sheet information and could point you toward where to remove it (sheet properties).
