I'm still struggling to figure out why ESR is relevant, or ever was relevant, for that matter. As far as I can tell, his most significant engineering accomplishment was fetchmail, and I'm not sure many here would call it very significant these days.
(I'm willing to accept the downvotes I'll likely get if only someone will answer me on this point.)
Apart from his being a great public advocate for open source software, he wrote "The Art of Unix Programming" and "The Cathedral and The Bazaar" -- both very influential.
Also ncurses (which is still widely used) and a large amount of GNU Emacs code. I cannot comment on how much of the code that is in use in those tools now is still his, but I don't think that matters. But probably his writing and advocacy for Open Source is more influential than his code.
(I'm willing to accept the downvotes I'll likely get if only someone will answer me on this point.)