Signal perhaps does not allow you to export your message history through the front door, however decrypting and exporting your message history is relatively low effort.

You messages are stored in encrypted SQLite3 database. The Signal encryption key is in

    ~/Library/Application\ Support/Signal/config.json

in plain text. If you have SQLCipher (https://github.com/sqlcipher/sqlcipher) compiled you can decrypt your Signal database:

Navigate to

    ~/Library/Application\ Support/Signal/sql/

and type

    sqlcipher db.sqlite
    sqlite> PRAGMA key = "x'<your_key_here>'";
    sqlite> .schema

and query away.

Of course there is a Python package to automate all of this here:

https://github.com/carderne/signal-export

This exports your message history as markdown and HTML files for your convenience and it will do incremental exports as well.

For iOS the same holds true, considering iOS has had a jail break most of its existence.

So, in retrospect your Signal messages are only as secure as computers of the people you talk to and of course your own device.

I would go a step further and assert that there is no such thing as secure communication.