This news was first reported by Sebastian Wenzel on Spielen.de, and I've confirmed with Ravensburger that Brück is leaving at his own request. The two alea titles teased at the Spielwarenmesse 2020 trade fair — The Castles of Tuscany and a new version of Puerto Rico — will be released as planned.
The alea game brand debuted at Spielwarenmesse 1999 with the presentation of Reiner Knizia's Ra and Karsten Hartwig's Chinatown. Brück had joined Ravensburger when that company bought his previous employer, F.X. Schmid, in the late 1990s, and the alea line was established to present strategy games somewhat more involved than the family games published by its parent company. While Brück is mostly known for his development work with alea, he has also edited many games that bear the Ravensburger logo.
I'll note that I've played games with Brück several times, specifically when designers were pitching potential alea titles to him. After a single playing, Brück would present his understanding of the game back to the designer, boiling everything down to its core, akin to characters in The Matrix seeing through reality to the code underneath it all. Everything was boiled down to points, money, bits, and actions — the game world in its most basic elements — and Brück would lay out the issues that he saw in the design, effectively inviting the designer to pitch the game again in a year when all of those issues had been worked out.
Ravensburger states that the alea brand will continue, although a successor to Brück's role as editor, developer, and line manager has not yet been named. I asked Brück about his future plans, and he said, "Give me some more weeks for a more concrete answer."