I need to add a native app install banner to my web app for Android users. I've followed these instructions. The banner is appearing for Google Chrome but not Samsung Internet.
Are native app install banners supported in Samsung Internet? If not, is support planned?
You have to follow Web install banner for web apps which you access in Chrome/Samsung internet. Not Native install banners, which are for apps written in native API using languages like Java.
Samsung Internet is built using Chromium and does have add to home screen option based on this doc. See if you are following the steps and publish your Chrome - Lighthouse PWA audit report if didn't workout.
Update: I got mislead by the PWA tag to the question and thought you were referring to Web install banner. related_applications which enables what you are expecting is a W3C spec that is in "Working Draft" status. Samsung internet is not even listed for compatibility and no work form Samsungs doc as well. We could only assume its not supported until it starts to work or Docs are updated.