/e/ is right now just a fork of LineageOS 14.1 with a bunch of degooglization and microG. microG isn't currently compatible with the latest version of GCM/Firebase. WhatsApp, for example, works around that but that might not be what you want; it is quite the battery drain that way. Although such push messages are as well (like IMAP IDLE is). Depending on your phone, you got LineageOS 15.1 already. Though you might not find the differences between 15 and 14 major. Though the news here seems to be that there's a new version of /e/ based on LineageOS 15.x