A full frame sensor is twice the size of APS-C and 4 time size of m43. So mathematically speaking there is twice and 4x the area for the dust to stick on respectively
But in reality I imagine they are just as dust prone as any other mirrorless camera. EOS R/P gets by this by closing the shutter when the camera is off. Recently Sony added this feature via. a fw update to A9II. They may or may not update other A7 bodies (the certainly won't update 2nd gen ones).
IMO its a non-issue blown out of proportion by the internet just like silly bigger
dick mount is better debate

But if it really bothers you there is the option of going the canon route or splashing out on A9II