Is "Ignore ownership on this volume" reversible?
Yes, ignoring ownership is reversible, but I wouldn't recommend that as being a solution to your problem because the Ignore ownership on this volume option shouldn't be used on a drive you also want to use as a bootable drive for one of your Macs. And, since your Macs are running different versions of macOS, you may be asking for added trouble. In addition, you mention that this is also a backup drive - while you don't clarify this, in the case of drives also used by Time Machine Apple recommends:


  Make sure "Ignore ownership on this volume" at the bottom of the "Sharing &amp; Permissions" section of the Get Info window is deselected (unchecked).


Source: https://support.apple.com/HT202380

Ignore ownership on this volume

The main reason the Ignore ownership on this volume option exists is to ensure that file servers, applications, operating systems, etc can still function correctly because they require permissions to keep track of which users have access to which files. 

However, drives used purely for data storage (as opposed to being boot volumes) shouldn't generally use permissions and ownership (although they can be secured in other ways). 

What I'm saying is that the Ignore ownership on this volume option shouldn't be used for boot disks or server volumes (and you may not even get the option anyway - depending on macOS version). It's best this is only used for drives that are used purely for data storage, and even then only for drives that are directly connected to your computer.