No, this is not what E2EE means at all. E2EE means the message is not readable in transit nor is it measured, scanned, sampled, copied, exported, or modified in any way without explicit action taken to do so by one of the legitimate parties to the conversation.
If the client just leaks the plaintext or leaks any information about the plaintext that encryption is supposed to protect then the encryption scheme cannot be described as "end to end".
Client dictates what ads are shown. Fb knows what ads are shown to who. Fb now can deduce what topics people are talking about. Technically convo info has leaked. If someone is getting served ads for Trump, they probably like Trump. If they are getting ads for Biden they probably like Biden. Etc…..
Yes, so that would violate the end to end principle. If the client downloaded all of the possible ads and the selection was totally local, and interaction with any of them was a user choice I think that could still be fairly described as E2E though. Or ads were fetched by private information retrieval.
If the client just leaks the plaintext or leaks any information about the plaintext that encryption is supposed to protect then the encryption scheme cannot be described as "end to end".