Here's something that will bake your noodle. Sage 100, the accounting/ERP software, is still to my knowledge written in Business BASIC from the 1970s—specifically the variant that ran on MAI Basic Four minicomputers (just 64 KiB of RAM!). This variant of BASIC was so, as Claude says, load-bearing that several companies emerged to support source-compatible versions on a variety of platforms as the non-mainframe business world evolved from custom-schmustom minicomputers to Unix workstations and PCs. The most common variants today are BASIS International's BBx and BBj (Java), and ProvideX. Sage 100, originally MAS 90 and MAS 200, was an accounting solution originally developed for those 1970s minicomputers and then forward-ported to new platforms as the business landscape evolved.