I was upset when I saw this and I went to silversneakers.com and I am still eligible for silver sneakers and could print out a card with my member number. You don’t need United Health Care to get the benefit.
Unfortunately you need an insurance provider who contracts with SS. I called and checked with SS. The reason the website says you're eligible for a free membership and gives you a card is because your 2018 insurance carries SS. Individuals cannot get a membership according to the representative I talked to at SS.
But you DO need a provider that carries Silver Sneakers. Yes, you can enter your information on the Silver Sneakers website and it will tell you that you are eligible for membership for free. But that's because your insurance provider has enrolled their subscribers for this year. I called SS and checked. There is no way to belong to SS without the insurance providing it to it's members.