Is the homebrew warning "We (and Apple) do not provide support for this old version." safe to ignore?

Solution 1:

I wouldn’t ignore it. It’s important - things likely won’t work as expected and you have to do extra diligence on ensuring you get security and stability patches. Same with running an unsupported macOS version but with homebrew, at least the source is open and easier to patch DIY than Apple tools are to tinker with the core OS.

• Does it mean you can’t proceed? No
• Should you think about it, and regularly check your risk level? Yes

Solution 2:

Yes, the warning in question is safe to ignore.

Homebrew will only install/upgrade to package version build specifically for your installed OS version.