Bug reported by @vasild here: (bitcoin-core/gui#86 (comment))

listen=0 will cause us not to listen on any port, so we would not accept any incoming connections, regardless of whether clearnet or Tor.

$ bitcoind
...
2020-10-28T08:41:57Z Config file arg: listen="0"
2020-10-28T08:41:57Z Config file arg: listenonion="1"
2020-10-28T08:41:57Z Config file arg: rpcport="8332"
...
2020-10-28T08:42:19Z tor: Got service ID foo, advertising service foo.onion:8333
2020-10-28T08:42:19Z AddLocal(foo.onion:8333,4)

$ netstat
tcp4 0 0 127.0.0.1.8332 . LISTEN
(bitcoind is only listening on the RPC port)

I guess this can be considered as a bug because it will (I guess) advertise foo.onion:8333 where nobody is listening.