Have you previously downloaded El Cap? If so, it should be in your Purchases tab on the App Store and you can download it from there.
You can also try to search for it on the App Store and download it, though I can't test at the moment to confirm if this will work.
I have purchased, or downloaded, all OS X and macOS systems, and can go on my backup system, running Sierra, and immediately download those older system installers --- at least those that will boot on my older Mac. None are available through the Mojave App Store. I can get an El Capitan information page, and the "Get" button takes me to the Software Update pref pane, where I see "Update not found. The requested versions of macOS is not available."
If I try the same to download Sierra, it doesn't even open an info page in the App Store, and simply shows "Cannot Connect to App Store"
There doesn't even appear to be a simple way to download High Sierra --- other than a python script that someone has posted that gives download access to High Sierra, particularly the 2018 MBPro version.
Bottom line --- can anyone say that there is ANY way to use your previous history in the
Mojave App Store to download ANY OS X or macOS system installer, including older systems that STILL will boot your existing Macs?