The Cubs are no strangers to selling at the deadline. Just last season, the team parted with stars Anthony Rizzo, Kris Bryant, and Javier Baez. These were players who, just like Contreras, won a World Series ring for the Cubbies. So, a move is definitely possible.
That begs the question, is a Contreras trade likely? It may not be. MLB.com pointed out that of all catchers with at least 300 plate appearances and a 2.0 WAR in a season dating back to 1967, only seven catchers having what they considered to be a season were traded midseason.
Furthermore, no team in history has won the World Series after acquiring a star catcher midseason. Historically, it doesn't seem as if a trade is in the cards. However, it isn't impossible.
Two of the seven catchers MLB.com considered as having "strong" seasons were traded in 2016 (Jonathan Lucroy) and 2018 (Wilson Ramos). And with the Cubs likely to be sellers, they may disregard historical precedent if the trade package is to their liking.