For open source messengers, you can check whether they actually encrypt your messages and whether the server has access to your encryption keys but what about WhatsApp? Since it’s not open source, you can’t be sure that the encryption keys aren’t sent to the server, right? Has there been a case where a government was able to access WhatsApp chats without reading them from the phone itself?
Given enough time anything can be decrypted, so, yes. The actual question is if they would have any interest in doing so given the large investment of time and resources required when they can simply hit you with a wrench until you give them the password to your device or in more enlightened Countries, just buy the data directly form Meta. You don’t control the server so there is no assurance of any encryption being secure beyond your chat not being interesting enough to justify the attention.
Relevant XKCD