If "open mic" includes non amplified, and so singers nights, singarounds, floor spots and the like, there are places where the "welcome" ethos is not applied. There have always been some UK folk clubs where guitars are very unwelcome (the usual basis being that they cannot be proper for folk music because they were not invented when folk songs where written - all guitar music being at best contemporary acoustic). I am told that the club run by the Copper family is very like this. If Bob Copper does not approve of it (I am told) no-one else had better dare to. A friend of mine tells me he accompanied himself there on guitar to do "the Cuddy Wren" (approx 1342, I think) and was told off for his contemporary approach.