Strider PS4 was a PS+ title several years back. I let my PSN+ lapse for a while because I haven't been playing online lately and the games of the month haven't been that good for a while. So now I don't access to the game until I either renew or just buy it. When you are with PS+ you can't buy games you got through it because they show as owned in your library.
And Warframe is free to everyone.