Too long for a comment, so I will post as an answer, although it is not exactly what you ask for, parts of this should be acceptable.
If you do not get a solution, which seems likely, because MS tended to keep their proprietary binary formats very private, don’t give up.
Just do what I would have done in the first place anyway; unless there is some incompatibility problem, which is unlikely, take a copy of the file, import that into PowerPoint, let PP convert it to PPTX and save. Process that with R or Python (you can also automate the import/save as operation).
If you don’t have access to PowerPoint, Libre Office has a good chance of getting the job done.
If that fails, I regularly buy Windows and Office license keys on eBay. Last month, I paid US $12 for MS Office Pro.
If that’s too expensive, it is easy to sign up as a Microsoft developer, you just have to propose a project that you want to develop using their tools and they will give you free Office 365 for a year (probably easily renewable).
If none of that works, then you are SOL and no worse off than when you started :-/
HTH, good luck :-)