Is it possible to get Keynote/Pages/Numbers for El Capitan?
Here’s the deal:


iWork requires macOS Sierra (or whatever the latest OS version is at the time) to “purchase.”
You can install the latest compatible version onto your computer, but only if you already have that iWork app associated with your Apple ID.


So the solution is to:


Get access to a Mac with macOS 10.12 Sierra installed.
Create a guest account with your Apple ID and use it to login to the Mac App Store.
“Purchase” Keynote, Pages and Numbers.
Now if you go back to your older Mac, these products will be associated with your Apple ID, and the Mac App Store will offer to install the latest compatible version.