In Mac OSX this is alt + ~
Is there an equivalent for Elementary? I didn't see it as an option in the key
Afaik, there isn't a keyboard shortcut for that, presumably because of how workspaces are implemented.
Workspaces provide an alternative for some workflows and though I make heavy use of them for different purposes I have not found them to be an efficient alternative to [Alt + ` ] for cycling through windows of the same application in my daily workflow.
I use a python script invoked by a custom keyboard shortcut as outlined in the code comments to accomplish this. This has worked well on Juno and Hera.