I'd like to add a couple of things to the answers provided by my colleagues.
First, v5.0.9 and v20.10 are both LTS (long-term support). Look at Ubuntu (already mentioned by Chris), it has a few currently supported LTS versions. By definition, LTS versions will get fixes, that's why v5 releases keep coming these days.
Then, v20+ has a lot of major changes, some were mentioned by Joao, and I'd like to list them all:
- Secure by default. v20+ expect to run with SSL, unlike v5, which is insecure by default. v20 can run in the insecure mode for non-production load, but it disables both authentication and authorisation.
- gRPC is the new norm. We want to phase out TCP and ATOM Pub protocols. gRPC clients are available for quite a few tech stacks, not only .NET. We have supported clients for .NET, Java, NodeJS, Rust, Huskel, and more might be coming.
- v20 doesn't use Mono, which still has quite a few internal issues. It makes v20+ more stable and more performant.