https://signal.org/android/apk/ and it appears to work without Google Play Services (though push notifications drain more battery iirc because they can't use the google push notification service)
I mean yes, but Signal needs to grow up and decide if it's going to find a way to play ball with business, or stick to it's ideological purity and weird cryptocurrency experiments (I use Signal solely for all my messaging now).
Supposedly Signal received ~$600k in donations in 2018 while spending ~4M in infrastructure costs[0], so OP is suggestion they do regular revenue generation via in-app features and whatnot instead of cryptocurrency stuff.