I've used eth0:xx style aliases for that for a decade with no problem. Do you know what the difference is?

Yeah, your eth0:xx is just a label (a tag of sorts) attached to the entry. Otherwise, they're exactly the same thing.

You can even add an address with `ip addr add' that will be visible to ifconfig.

Yep. It was kind of funny watching a couple graybeards trying to figure out how keepalived was assigning addresses and how to view them. It’s a rare treat when you can help someone like that with something they might normally help you with.