For forty years, basically. The first "tugboat" TeX newsletters were written during the rewrite to "TeX82", so most people porting during that time were basing

For forty years, basically. The first "tugboat" TeX newsletters were written during the rewrite to "TeX82", so most people porting during that time were basing their variants on the the version written in SAIL ("TeX78"). It's quite fascinating to read and look at. Some from scratch rewrites in different languages, while the core group writes about the new version in Pascal (IIRC the literate programming tools were invented during that time). Old time-sharing and mainframe setups most people haven't heard of. Rewrites in C an Fortran. And still even that early TeX used there isn't that much different from what's still used today, including the DVI format, Metafont etc. (Both syntactically and when you see the output)