ntfy is a client for the UnifiedPush protocol, an OSS alternative to Google's firebase cloud messaging. It provides push notifications for compatible apps, this way instead of having each app wake up periodically to check for updates over the network, only one app (ntfy) keeps running the background and wakes the appropriate app when it gets a new push.
I've never used /e/OS but given the URL in your screenshot I guess murena uses UnifiedPush to send push notifications to its system apps. Blocking it may prevent you from getting system update notifications and such.