How decentralized is Farcaster today?

Sign ups:

You can sign up permissionlessly with the smart contract as a developer.
You can sign up with Supercast, Herocast, Farquest, and possibly others.
Data / APIs:

You can permissionlessly run a Hub.
There are thousands of independently run Hubs.
If you don't want to run a Hub, you can use an infrastructure provider like Neynar or Airstack.
Protocol vs. non-protocol features:

On Hubs: casts, reactions, follows, profiles, verifications.
Not on Hubs: channel metadata, settings and follows, direct casts, location in user profiles.
If Warpcast labels me as spammy, is that censorship?

Depends on your definition! If you have an existing audience — say 10K followers — then even if you get labeled spammy, your existing followers will see your content in feeds on Warpcast regardless of whether you use Warpcast or not.
If you don't care about Warpcast (approximately 80% of signer activity), then your follow graph is already decentralized in the protocol.