I’m not sure how much legacy code is written for contact apps in smartphones. But given the amount of tweaks and changes apple and Google make in each of their releases I would expect they wouldn’t have any problem integrating this feature.

Also you mentioned legacy code, but why T9 dialing isn’t a thing anymore coz that would be available if they were simply extending from legacy codebase.

Alright I’ll provide a use case scenario. I wanted to this functionality when I was in uni where a lot of my contacts had more than one role to play and I had trouble remembering all the roles my contact would have. Let’s say one of my contact was my coursemate but also worked in organizing committee of an event. I knew other members of the organizing comittee as well. So in the event that I needed to share an information with the entire organizing committee, I needed to remember committee list members instead of software helping me. And what about another contact who also part of the event but was on a different team like Public Relations. You could say just another tag would suffice no need for nested ones. But this kind of events happens multiple times and I would have to create multiple tags for multiple events (instead of nesting) and it becomes cumbersome managing the list.