System Nav Bar Required? If not, pros/cons for keeping it?

Haven't worked on an Android app in a bit and was wondering if the system navigation bar needs to be there or not.

Looking at my husbands phone (I am on iOS), it's not on any of his apps. Is there a new best practice in this regard? I was looking in Google's Material Design guidelines and it doesn't say it's a mandate. Moreover, the Android Developers Guide shows ways to hide it.

I posted this on some Slack channels I am on and some designers are saying it's required while others are saying it's not, so I'd love tor get your thoughts too.

If anyone has any notes on whether this should be kept or not for an optimal experience please let me know. Many thanks!