I’m not sure exactly where the line is drawn between “reality show” and “game show”, but Jeopardy’s celebrity tournament usually has some significant names in it. I remember one episode not too long after the first Iraq War with General Schwarzkopf as a contestant (unsurprisingly, he blew away the two TV stars he was matched against).
Donny Osmond did the show while flying back and forth from Las Vegas where he was doing a show. It was an 11 year run.
Wayne Brady hosts Let’s Make a Deal and is on both Whose Line Is it Anyway and Black Lightning.
Drew Carey hosts The Price is Right.
Patti LaBelle and Gladys Knight are legends - even though they’re retired they’re still celebrities.
Since you brought up the Masked Singer, it’s the one case where I kinda sorta get the OP’s point.
With something like DWTS, even if you don’t know all the celebrities, it can still be fun to watch, and you get to know everybody as the season goes on.
But the whole concept of Masked Singer is to spend weeks trying to guess who the celebrity is. Then when it turns out to be somebody you never heard of, it’s a big anticlimactic letdown.
(For the record, I don’t agree that being past one’s prime invalidates one’s celebrity status.)
Nelly isn’t a celebrity?
I once read that these shows take the biggest celebrities who have nothing better to do for 6 months.
Zendaya was on Dancing With the Stars before she hit it big. She was a Disney kid when she was on the show and has since broken out in a big way.