I follow this market a little, and haven't seen one yet. The brands I'd check would be Nokia and Doro. There was a company making small power bank and feature phone combo but I can't remember the name.
It might be more fruitful to look at mobile broadband hotspots or whatever they're being called now. Could imagine a 5g one of those would be more likely?
There’s Punkt (4G though) https://www.punkt.ch/en/products/mp02-4g-mobile-phone/
There's not, that I'm aware of, at least in the US market.
And using a feature phone for security reasons is laughable. Feature phones are notoriously insecure.
If you need tethering, just use a 5G hotspot.
Why are we using the term “feature phone” to describe phones that have unusually few features?
Why 5G feature phone ? I assume you want 5G purely for future proving ?
For anyone interested, there is a database here : https://dumbphones.pory.app/