Yes.
There is a document cross referencing old Decisions with new Rules/Clarifications indicating if or how they have changed.
Mapping Summary Chart.
https://www.usga.org/content/dam/usga/pdf/2021/rules/Mapping%20Summary%20Chart%20-%20FINAL%20-%20revised%209-1-2021.pdf
As it happens 19-1/1 has...
Yes. After the EG (men) switching to the USGA rating system but just prior to WHS, the USGA CR was rounded to produce the SSS. All the other CONGU unions had been doing it for years.
Slope is in effect a comparison/relationship between the Course Rating and the Bogey Rating
In fact in the 'leadup' to WHS all Course Ratings in England (for men) were done using the USGA rating system as opposed to the EGU SSS system. The resultant CR was simply rounded and called the SSS. The Bogey Rating (and Slope) were simply stored 'til WHS came in.
Everywhere else in CONGU had...
Scotland (and I think Ireland) adopted the USGA Course Rating system (including Slope) many years before the rest of CONGU but couldn't use it with the old UHS because the then EGU refused to contemplate it. They thought their (no Slope) system was sufficient. I believe Scotland (and Ireland)...