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Hey Jakub,
There are no planned updates for rekordbox 5. If Big Sur breaks it, we will not be fixing it, but rather recommending users stay on Catalina or older.
As for rekordbox 6 working with Apple's new hardware, I know there will be a transition period - whether that means the Rosetta translation works or if a rewrite of the software is required, we'll eventually get to a point where rekordbox 6 functions properly on the M1 chips.