There has been lots of talk about what thread to use for appliqué. I've been appliquéing since the mid 90's. Figuring out what I liked best took me a bit. My first instructor liked a Mettler polyester. So following her advise, I used that. While this thread might be good for other uses, it did not work for appliqué. It tended to knot quite a bit. Frustrating.
I continued on with appliqué. I loved it. I learned from others here and there. One of my instructors liked silk. OK I was game. I bought some silk. It was supposed to disappear into the fabric. Mostly it just disappeared from my needle. It also stretched and left an irregular edge.
Then someone introduced me to cotton. So I tried cotton. I began to use Mettler 60/20 or DMC 50 wt. cotton. This disappeared into the fabric very nicely. I liked it. I used both of these threads for many years. Then Aurifil came up with a very nice cotton thread in a number of colors. I really enjoyed using this thread.
There is an opinion, that over time, cotton thread is best on cotton fabric because silk or poly will bite into the cotton as it ages making for faster deterioration of the quilt.
This may prove true -- or not. It certainly isn't an "urban legend" as some merchants of silk and poly thread would have one believe. But are there studies out there? I just don't know. Perhaps some reader will find a study and point it out. I haven't made it my life's work to look into it.
I progressed in my appliqué journey trying various threads along the way. I found that I like the feel of cotton the best so I've stuck with that. Will my work outlast the work of you silk and poly users? I have no idea and worrying about how long my work lasts isn't keeping me up at night. It would be nice if my work is still around 100+ years from now but that is not why I choose to use cotton thread. I appliqué, piece, and quilt for the pleasure of it. I love the feel of cotton thread.
I will continue to try new threads as they become available. Right now, Aurifil is my go to thread for appliqué. I also like DMC 50 wt. Mettler 60/20 is another good thread for me. I also like Madeira Cotana 80 wt.
Conclusion. Do what pleases you but try them before you make a decision.