This is a little unusual, as most of us are. 1-year, almost to the day, my 1-130 for my wife was approved. So I'm now working on the I-864. This is our second time through I applied 8 years ago and she got a green card, along with my stepson. She gave hers up when we moved to Portugal. He didn't. Now he lives and works in the US.
In the I-864, Part 3, it asks about the Immigrants I'm sponsoring. Since I'm still liable for his support, it seems like I'm still sponsoring him. But he's already in the US; not coming within 6 months or after 6 months of my wife. I assume it means Who am I Sponsoring on this I-864 because in part 5 (Sponsor's Household size) it asks about other people I have sponsored, previously.
So, I believe I should ignore him for part 3 but include him for part 5.
Another interesting thing is when it asks for my physical address it has a space for country. But I thought only people residing in the US were allowed to sponsor family immigrant visas.
Having done this twice, and looking at the changes USCIS has made, I can see they've made some things simpler. But I've gotten older and less capable, so its a wash.