Based on the comments so far, maybe something like this makes sense:

Warning: Private messages in Lemmy are not End-to-End encrypted, so the respective instance owners are technically able to read them. Please use a platform with E2E encryption for private messaging. Lemmy recommends Element.io and XMPP.

I agree. That’s why I propose to clarify the wording.

Yes. And I think saying “messages in Lemmy are not End-to-End encrypted” is clearer communication than “messages in Lemmy are not secure”.