Download Correct Version of Numbers for Mac

If you have not installed Pages since October 2013 from the Mac App Store, you cannot get the last compatible version for any macOS version older than Sonoma but you can try my theory in the fourth paragraph here.

If you have purchased Pages at any time since October 2013, open the Mac App Store & hold down the option key while clicking the Purchases tab on your Mojave Mac. If you see Pages, Numbers, and Keynote with Update/Install buttons, it may offer to download a previous version of these applications compatible with older OS versions.

In Mojave and later releases of macOS, one must enter ⌘+0 (zero) to get to the previous purchases panel in the Mac App Store. This changed from the option+Purchases tab in High Sierra and earlier.

I have not been able to test this method so it is just a theory but someone posted in the Numbers forum that it does, indeed, work to get the post-2013 versions of the iWork apps. First, you need access to a computer running macOS Sonoma or Sequoia (as of this date). Perhaps a friend will let you do this. Then you need to sign into the Mac App Store with your Apple ID & "purchase" the apps. Yes, they are free, but the process to acquire them & get them associated with your Apple ID is considered a purchase. You will then need to sign out of the Mac App Store on that Mac (unless it's yours) & delete the apps. Then sign into the Mac App Store on your older Mac & it *might* allow you to download them as described in VikingOSX’s reply in this discussion in the Pages for Mac forum. 

See VikingOSX’s reply in this discussion in the Pages for Mac forum.