According to Plexus, WhatsApp works perfectly on Android without Google Play Services, whether or not you have microG installed.[1] I think they implement their own push notification system if you download directly from them,[2] though I haven't confirmed this.

Discord works perfectly with microG, and has a 3/4 rating without it since notifications will only work if you have microG.

[1] https://plexus.techlore.tech/applications/whats-app

[2] https://www.whatsapp.com/android/

[3] https://plexus.techlore.tech/applications/discord