With Android phones you could possibly use an app like NoRoot Firewall to cut off net access for the app when you want to appear offline. Unfortunately that's a pretty "advanced" use of apps for the average consumer and definitely not a replacement for building this in natively.
Depends on your reasons for showing offline. If offline is a proxy for “I am not here to check and respond to messages”, such as when you’re sleeping, quite useful. If not, and you’d prefer to be constantly offline for privacy reasons, less useful.
You don't "show offline" in WhatsApp, either "online" appears below the contact name or nothing appears. Nothing doesn't mean offline, it means the person doesn't have WhatsApp running in the foreground, they could or could not be online (which most define as "using or not using the phone right now").
