Alfred 4 is officially supported on macOS 10.11+, but is compiled to work on macOS 10.10+.
In addition, Alfred is built on top of the macOS metadata which is maintained when Spotlight is enabled (by default) on these versions of macOS. For Alfred to be able to serve you fully, you should not turn off Spotlight indexing.
We aim to always support the most recent versions of applications we integrate with (like 1Password and iTunes), so recommend that you stay up to date too.
Alfred 3 is available for users on macOS 10.9+, should you wish to download it.
Alfred 2 is available for users on 10.6+, but is no longer supported.