The phrase "popular mainstay" means something that is well-liked and has stayed consistent and widely used or enjoyed over a long period of time. Full definition
Some of the themes he introduced, such as the Hyrule Castle, Kakariko Village, Zelda, and Ganondorf themes, have become popular mainstays of the series. (squareenixmusic.com)
Reservations are available in popular mainstays like Los Cabos and Cancun, too. (seasidemexico.com)
Copycat apps designed to resemble popular mainstays remain a popular method of trying to deceive users, according to Ahn. (theverge.com)