It does have a backup functionality, which unlike WhatsApp's doesn't store your messages unencrypted. It also doesn't force you to use a specific storage provider. If you want to have it synced, you can point your favorite sync client at it.

https://support.signal.org/hc/en-us/articles/360007059752-Ba...

Additionally, they've introduced an optional PIN which you can use to recover profile, contacts and settings when switching devices - without the need to setup sync yourself.

https://support.signal.org/hc/en-us/articles/360007059792-Si...