Ter Stegen can't continue next season at Barça or, at the very least, this is what those in Camp Nou offices believe. In the coming hours, the signing of Joan García will be officially announced, as he's coming to take his place in a World Cup year. This has been a very carefully considered decision, but with arguments behind it and with the express approval of coach Hansi Flick.
Reasons for Ter Stegen's farewell
Journalist Xavi Torres explained that, in the days leading up to the second leg of the Champions League semifinal against Inter, Ter Stegen's agent called Deco. In this conversation, the captain's representative wanted to make it clear to the sporting director that he was absolutely ready to be the starter. In this regard, he argued that he had recovered from his injury and they'd have all the sporting guarantees if he played.
Deco relayed this call to the coaching staff led by Hansi Flick, who decided that Szczesny would continue until the end of the season for two reasons. One, his good previous performance and, another, Ter Stegen's lack of match fitness.

Ter Stegen didn't like this decision at all, and he declined the possibility of traveling with the rest of his teammates to Milan. As captain, he didn't want to be part of the group and stayed in Barcelona training alone. Other injured players like Koundé chose to catch a plane to support their teammates.
Ter Stegen's poor attitude, combined with the pressure he exerted both publicly and privately to regain the starting spot, somewhat soured the atmosphere in the locker room at the end of the season.
"They didn't like it at all"
This doesn't end here. Ter Stegen also made sure to give a lengthy interview to a media outlet in his home country, Germany. In it, he made it abundantly clear that he was ready to return. This was a public message that he also conveyed privately to Hansi Flick.

Even before this, the relationship between Ter Stegen and Hansi Flick wasn't very good since Flick's time as Germany's coach. Back then, the coach decided to go with Neuer in goal, something that didn't please the Barça goalkeeper at all.
The attitude Ter Stegen showed in the final weeks of competition "wasn't liked at all," nor were his attempts to apply pressure. Barça is ready to move on and hopes that, with the World Cup to be played in 2026, the German will decide to leave. Hansi Flick's choice is clear: his starter is Joan García.