Any "secure" so that relies on someone else for security is not secure.

Fuck the scope of E2EE.

When someone has FSB/NSA agent behind them reading messages, no amount of encryption will help. Biggest cybersecurity vulnreability is located between monitor and chair. When you are texting someone else, that someone else's chair-monitor space is also vulnreable.

Signal makes a lot of claims on their website that are laughable.

Well, maybe. I didn't read their claims, nor I use signal.

Attachments are stored unencrypted, keys in plaintext.

Is OS-level encryption plaintext or not? If yes, then they are encrypted, provided user enables such feature in OS. If not - nothing if encrypted fundamentally.

If they were serious about security, they would depricate the windows app and block it from their servers.

WTF does Apple have to do with anything?

You just used applefans' argument. Yeah, I wonder what.