I think the core meaning, without UD's connotations, is someone who's present, but doesn't really participate. I believe the word comes from people who go to so

I think the core meaning, without UD's connotations, is someone who's present, but doesn't really participate. I believe the word comes from people who go to social events like dances but instead of going out on the dance floor to dance with everyone they stand along with walls and watch. "Shy" is not the right word for the context described by Rob Pike. Presumably the notional wallflowers at a tech conference or meeting would be people who didn't feel like any attempts at participation would be welcomed, taken seriously, or given due consideration. The non-wallflower astronomers are full and equal participants in the discussion.