The defense against that is 'disappearing messages' which is available in most popular E2E messaging apps nowadays, including Signal and WhatsApp.[1]
PGP emails doesn't even have forward secrecy. Emails are not messaging, it needs video/voice calls, stickers/gifs etc etc to have any hope of being adopted by non-techy folks.
The Signal blog has a number of articles on how they develop state-of-the-art privacy preserving features. [2][3][4][5][6].
Also the only info Signal has about you is "Unix timestamps for when each account was created and the date that each account last connected to the Signal service", which is what it provides to government requests [7].
[1] Disappearing messages
https://signal.org/blog/disappearing-by-default/
[2] How to build large-scale end-to-end encrypted group video calls:
https://signal.org/blog/how-to-build-encrypted-group-calls/
[3] Signal and GIFs
https://signal.org/blog/giphy-experiment/
https://signal.org/blog/signal-and-giphy-update/
[4] Signal groups,
https://signal.org/blog/signal-private-group-system/
[5] Sealed sender
https://signal.org/blog/sealed-sender/
[6] Private contact discovery
https://signal.org/blog/private-contact-discovery/
[7] Government requests
https://signal.org/bigbrother/