Cat Spencer still runs into people who want to talk about Radix Gallery, the downtown Phoenix art space she ran more than 30 years ago. “I did a pop-up show in Scottsdale for Cliff Benjamin a while ago,” Spencer said last week from one of her homes in New York. “People kept coming up to me to tell stories about Radix and the art they bought there.” Spencer’s gallery was a late-’80s anomaly: a refined, contemporary gallery that showed conceptual art by up-and-comers. Local art history was made there: The first meetings for what would become downtown’s monthly art walk were held at Radix. Until very recently, and despite the fact that the gallery closed in the early 1990s, its name remained on the structure’s façade.