It was somewhere, but I don't remember where exactly. Basically the IBIS will work with IS in EF lenses, but it won't be 'coordinated', so not as good as with RF lenses with IS.
This is interesting I found it on digitalcameraworld.com re IBIS on both R5 and R6 how the none IS 28-70 f/2 and 50 and 85 f1.2 get 8 and 7 stops of IBIS even thought the lenses don’t have IBIS.
No mention on various EF lenses but it looks like it will differ lens to lens
Lens Lens IS only Lens IS + in-body IS
RF 24-70mm f/2.8L 5 stops 8 stops
RF 24-105mm f/4L 5 stops 8 stops
RF 24-105mm f/4-7.1 5 stops 8 stops
RF 28-70mm f/2L None 8 stops
RF 70-200mm f/2.8L 5 stops 8 stops
RF 85mm f1.2L None 8 stops
RF 15-35mm f/2.8L 5 stops 7 stops
RF 35mm f/1.8 Macro 5 stops 7 stops
RF 50mm f/1.2L None 7 stops
RF 24-240mm f/4-6.3 5 stops 6.5 stops
RF 100-500mm f/4.5-5.6L 5 stops 6 stops