What happens if I opt out of Signal Backup? I was just offered the new feature

I just got a notification that Signal back up is now available to me. I understand there’s a free and paid option.

But how did Signal backups work before this launch? Were all messages automatically saved? Or was it always just the last 45 days?

Also, for those who choose the paid plan, is there an option to pay anonymously?

Because I don’t use the Google Play Store on my phone. My Android phone isn’t linked to any Google account.