**TL;DR**: When upgrading from `Bitcoin Core 0.20.1` to `Bitcoin Core 0.21.0`, a downstream application using the HTTP JSON-RPC API started getting HTTP 403 "Forbidden".

Turns out it is because I have `rpcallowip=::/0` in `bitcoin.conf` which used to work but does not anymore.
As a workaround, `rpcallowip=0.0.0.0/0` works.

-----

I run bitcoind in docker container1, and some application using the HTTP JSON-RPC API of bitcoind is in docker container2. They are on the same "docker network".

In `bitcoin.conf` I have:
```
rpcauth=<...>
rpcbind=0.0.0.0:8332
rpcallowip=::/0
```
(bitcoind RPC is not actually exposed to the public internet; it is up to the docker layer and above to ensure this.)

The application in container2 is connecting through HTTP accessing an URL such as `http://user:pass@172.19.1.10:8332/`.

This setup used to work when running `Bitcoin Core 0.20.1` in container1.

It no longer works with `Bitcoin Core 0.21.0`.

-----

If I change my `bitcoin.conf` as below, it starts working again using 0.21:
```
rpcauth=<...>
rpcbind=0.0.0.0:8332
rpcallowip=0.0.0.0/0
```

So `rpcallowip=::/0` used to work but does not anymore; `rpcallowip=0.0.0.0/0` works.

-----

Easy way to test:
`curl --data-binary '{"jsonrpc":"1.0","id":"curltext","method":"getblockchaininfo","params":[]}' -H 'content-type:text/plain;' http://user:pass@172.19.1.10:8332/`
CBlockPolicyEstimator* fee_estimator = node.fee_estimator.get();
        node.scheduler->scheduleEvery([fee_estimator] { fee_estimator->FlushFeeEstimates(); }, FEE_FLUSH_INTERVAL);
    }
     
    for (const std::string port_option : {
        "-port",
        "-rpcport",
    }) {
        if (args.IsArgSet(port_option)) {
            const std::string port = args.GetArg(port_option, "");
            uint16_t n;
            if (!ParseUInt16(port, &n) || n == 0) {
                return InitError(InvalidPortErrMsg(port_option, port));
            }
        }
    }
    for (const std::string port_option : {
        "-i2psam",
        "-onion",
        "-proxy",
        "-rpcbind",
        "-torcontrol",
        "-whitebind",
        "-zmqpubhashblock",
        "-zmqpubhashtx",
        "-zmqpubrawblock",
        "-zmqpubrawtx",
        "-zmqpubsequence",
    }) {