I don’t buy into silicone sealer killing O2 sensors for one second. As I mentioned, by the time someone is gooping up oil pans, valve covers (possibly in tandem with a head gasket), and so on that tells me someone is dealing with a high miles or problematic engine. Any O2 problem is likely related to the engine problem.
Now why would sealant manufacturers have a disclaimer? To weed out the possibility of the ever present lawsuits and sniping where someone claims that silicone killed their 150k miles oil burning motor.
Look at the mountain of grief that Fram goes through over countless internet stories about their oil filters being faulty.
Class actions suits are filed for a lot less reason than the above. My mind is subject to change if someone could provide one verifiable case of an O2 being murdered by silicone.