download macos installers without installing

Is your Mac running High Sierra, so if you are trying to download another OS that is older than High Sierra

the App Store refuses it, the App Store thinks you are going to install it, it doesn't;t know that you are saving it for another Mac.

What OS is it you want to download for the older Mac, macOS's from El Capitan backwards if they had been downloaded previously will appear in the App Store Purchased section. If this is the case for the other Mac what you can do is use your Mac and login to the App Store using the Apple ID for the other Mac and see if you can download whatever OS you are wanting.

Why can't you hook up the other Mac to the internet.

Open the System Preferences and click on Software Update. Click the Advanced button. Use these options.

With the first option checked, you'll only receive notifications an update is available. It's then up to you whether or not you want to install it, and when.

Make sure the last option is on. This provides background security patches that can be handled by Apple without the need to inform you. Larger security patches are provided in updates.

Download from the older Mac. Use Recovery there to install the oldest permissible and if that’ll even download, or otherwise roll forward to the older release that you want.

Apple is reasonably trying to reduce the number of folks that get themselves in trouble with these downloads, and is seemingly also deliberately making it harder to get at various of the older releases.

Folks that do succeed then install those older releases, and that then run into old bugs and old limits and old security, etc., and then ask here in the forums and (undoubtedly) also ask Apple Support about these old bugs, and old limits.

We’re on a treadmill of upgrades with our increasingly networked and interconnected systems, and staying on old releases is increasingly difficult, at best. The old installer wasn’t saved, which might mean there are already gaps in the never-upgrade plans, too.