**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/`
GetMainSignals().FlushBackgroundCallbacks();
    
    if (g_txindex) {
        g_txindex->Stop();
        g_txindex.reset();
    }
    if (g_coin_stats_index) {
        g_coin_stats_index->Stop();
        g_coin_stats_index.reset();
    }
    ForEachBlockFilterIndex([](BlockFilterIndex& index) { index.Stop(); });
    DestroyAllBlockFilterIndexes();
    
    
    
    
    
    if (node.chainman) {
        LOCK(cs_main);
        for (Chainstate* chainstate : node.chainman->GetAll()) {
            if (chainstate->CanFlushToDisk()) {
                chainstate->ForceFlushStateToDisk();
                chainstate->ResetCoinsViews();
            }
        }
    }
    for (const auto& client : node.chain_clients) {
        client->stop();
    }