We know the technology exists. Apple had it all polished and ready to go for image scanning. I suppose the only thing in which we can place our faith is that it would be such an enormous scandal to be caught in the act that WhatsApp et al daren’t even try it.
(There is something to be said for e2ee: it protects you against an attack on Meta’s servers. Anyone who gets a shell will have nothing more than random data. Anyone who finds a hard drive in the data centre dumpster will have nothing more than a paperweight.)