From scratch OS installation on old iMac

Years ago, due to a Mavericks OS bug, I had to reinstall the OS on my late 2009 iMac. From what I recall, I had to go back to the original OS (Snow Leopard, I think) and update the OS from there. I'm on High Sierra OS now and wondering if I should keep all these old OS installers around in case something like that happens again. Or would I be able to download High Sierra (or some other version of the OS) for free and install it directly?