indexing speed probably wouldn't be a problem. the index size would probably exceed available disk space for a typical workstation of the time.

The wiki page says ctags was part of the 3BSD release, which would put it about 1979. My memory isn’t good enough to recall which release of emacs picked up support.

I think there was a find-function key binding for lisp code before ctags, but these are very rusty bits I’m recalling.