yes, in mid-2021 he had a serious cycling accident in Croatia in which he suffered a fractured skull, he was away from the trackside (obviously) and in that phase Red Bull suffered a bit setup wise, just to come back on form when he recovered
here's a random sentence from Marko after Turkey 2021: “He is now not fully fit but is ready to do his job and has immediately recognised the difficulties we had here [Turkey] in terms of set-up,” Marko added.
I don't know how involved he was in the design and development of RB20, I think this will remain a mystery for long time, but he surely was an allround inspiring technician whose absence is hurting.