If you're willing to compile plugins yourself, you can use it with any engine version you want. The only limitation is that plugins utilize Programming Subystems introduced with UE 4.22.

GitHub repository contains branches even for yet-unreleased engine versions. However, pre-compiled plugin releases are only possible for Unreal Engine versions released via the EGS launcher. The list below lets you know which engine version is supported for a given Flow Graph version.

• UE5 Early Access - upcoming Flow Graph v1.1
• UE 5.0 - planned a minor plugin release just after UE 5.0 release

• UE 4.27 - upcoming Flow Graph v1.1
• UE 4.26 - v1.0+
• UE 4.25 - v1.0, v1.1. Probably not supported in next releases, as the new major feature #15 Implement Asset Search for Flow assets/nodes might require 4.26.
• UE 4.24 - v1.0-1.1. Not supported in next releases.
• UE 4.23 - v1.0-1.1. Not supported in next releases.
• UE 4.22 - v1.0-1.1. Not supported in next releases.

Older engine versions got abandoned when supporting given versions become too much hassle. It's usually caused by engine changes related to the writing editor module. And some new features require a recent engine version, can't go around that.