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No it isn't decentralized.
I think some is opened sourced.

What would make it decentralized?
It does have witnesses?

Would require quite a few changes, but just because it has witnesses doesn't make it decentralized. Contracts (code changes) can be done without any witnesses approving it. The gateway in / out is completely centralized (hive.swap for example).

So, no 17 witness supermajority coded into the side chain changes?
It all prints in json on this chain, so it should be immutable?

I've used some of their gateways with no issues.
Don't much like the 1% tariff each way, but craptitalism kinda requires that, if Aggie wants to eat.