What did the sticky bit originally do when applied to files?

In various places one can see the "sticky bit" accused of nowadays being a complete misnomer, as its functionality _nowadays_ is to affect the write permissions on directories and act as a _restricted deletion_ flag. In an AskUbuntu answer the answerer wrote that ["a sticky bit usually applies to directories"](https://askubuntu.com/a/789948/43344) . I observed that indeed modern systems seem in practice to never apply it to files, but that a long time ago the _usual_ case was for it to apply to (executable program image) files rather than to directories. (When it comes to the paucity of modern usage on files, there's a related question at https://unix.stackexchange.com/questions/23757/ .) This prompted the question: > [What _did_ a sticky bit applied to an executable do? Was it like setuid then?](https://askubuntu.com/questions/789938/why-do-mount-ping-and-su-have-a-sticky-bit-set/789948#comment1187830_789948) Note the past tense. This is not https://unix.stackexchange.com/questions/79395/ now. It's how it used to work then.