Recently there's many 'Prologue' games on Steam, demos on GOG, some each with their own achievements.

https://store.steampowered.com/search/?term=prologue

Are speedruns of these accepted? Some have their full games out while some release in 2021 etc.

Prologues are probably fine as they are usually full games in their own right, assuming they fit all the other requirements. I think there's now rules against demos because of them having less content but it's not explicitly banned in the Request Game rules that I found so I may be wrong there. Check for yourself.