Right you are, thanks.

But there is no "privacy policy" on the Samsung keyboard about page.

The Samsungs privacy policy on the web states this:

Samsung Keyboard information: The words that you type when you enable “Predictive text”. This feature may be offered in connection with your Samsung account to synchronise the data for use on your other Samsung mobile devices. You can clear the data by going to the “Predictive text” settings.

Source: https://privacy.samsung.com/privacy/samsung

I wouldn't doubt if it's changed a few times. The phone was a 2016 Galaxy J3 V. I no longer remember the name of the 3rd party they were using for predictive text at the time, but I know that in the past they've used SwiftKey and Grammarly