Compatibility with Bitcoin Core v0.21 was implemented in a6878c8, but this did not get released yet.

If you're running into this error:

JSON-RPC error: JSON decode error: missing field `addnode`

You can add the following to your bitcoin.conf file to re-enable the deprecated RPC fields:

deprecatedrpc=getpeerinfo_addnode
deprecatedrpc=banscore
deprecatedrpc=whitelisted