I just got an email from Retirement Travelers, a YouTube wife and husband who are spending their retirement traveling around the world. I think Tammy was the one who first recommended them to me. They have lots of good ideas and videos.
Anyway, they just posted a article listing 26 airlines that offer free long-term stopovers when you book with them. The stopovers start at 2 nights but most are more than 7 nights, some go from 3-6 months and one has no time limit. Some are advertised, but many are not. The article gave me some great ideas for future trips!
https://retirementtravelers.com/the-airlines-that-allow-free-stopovers