Couldn’t one just do PGP over WhatsApp, over Telegram or whatever? Instead of sending a plaintext message you’d send the PGP message but using existing infrastructure. Then there’d only be metadata to harvest but content should definitely be secure.
And nowadays, every popular IM network wants 3rd-party clients off their net at all costs. Signal is on the better side of the spectrum here. While they explicitly say they don't support non-official clients connecting to the official network, this isn't rigorously enforced. WA, on the other hand, would permaban your whole account if you even so much as try to use 3rd-party client.