**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/`
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    if (node.chainman && node.chainman->m_thread_load.joinable()) node.chainman->m_thread_load.join();
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    node.peerman.reset();
    node.connman.reset();
    node.banman.reset();
    node.addrman.reset();
    node.netgroupman.reset();
    if (node.mempool && node.mempool->GetLoadTried() && ShouldPersistMempool(*node.args)) {
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    if (node.fee_estimator) node.fee_estimator->Flush();
    
    if (node.chainman) {
        LOCK(cs_main);
        for (Chainstate* chainstate : node.chainman->GetAll()) {
            if (chainstate->CanFlushToDisk()) {
                chainstate->ForceFlushStateToDisk();
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