TLDR: it works depending how strong the case magnets are

I upgrade from 8 plus to 15pm and got a MagSafe case for the 15. Was also curious because a few months back, I needed a new case for my 8 and saw they offered it as well. I didn’t get one in the end but when I got my 15 I put it on my 8 and tested it out. It worked at least with the MagSafe battery bank I bought. It would have come in handy a month back when I was out of the country and lost my cable. My cousin was with me and only had a MagSafe bank. My phone was dead and needed to charge it by keeping the bank and phone still to keep the contact while we were riding a bus for hours. A MagSafe case on my 8 would have helped in that situation.