As the Cardano mainnet, will the Cardano testnet be maintained indefinitely and immutably or it will perish at some point in time?
No, there are plans to reset the testnet in the future, probably sometime after Goguen.
But it is immutable forever, if you keep the blockchain data.
Since it could be reset that means it's being maintained by a central entity or does it run on top of volunteer nodes? May 1, 2021 at 19:40
1It is run by the IOG DevOps' team, but most of the nodes are run by the community. May 1, 2021 at 21:18
Well, we can assume that it will be maintained indefinetely because Cardano will also will be under development.
In the same way as in system administration you have Dev, Staging and Production environments in Cardano we also need to have testnet before pushing that in mainnet.