From https://github.com/monero-project/monero/blob/master/docs/ANONYMITY_NETWORKS.md it seems you don't have to specify
If desired, peers can be manually specified:
--add-exclusive-node rveahdfho7wo4b2m.onion:28083 --add-peer rveahdfho7wo4b2m.onion:28083
Notice it says, if desired, not required. But from @jtgrassie's answer at How do you setup a node to communicate with other nodes only over an anonymous network?:
If you add one or more nodes that are on anonymous networks, yes, they will share their peerlists of other anonymous nodes they have connected to.
This makes me think that
--add-peer might be necessary to discover anonymous peers. However, this answer was from March of 2020, so a lot might have changed from then.
Do clearnet peers also provide anonymous peer addresses to other clearnet peers?
Edit: At the time of this writing,
--add-exclusive-node is no longer necessary to obtain tor peer seeds in mainnet since there are some default peers added in the source (see https://github.com/monero-project/monero/blob/f9d789d68ccca9a9b4aba483b7faa03a062c50c3/src/p2p/net_node.inl#L856-L859). For stagenet seed peers, add peers manually by finding them here https://monero.fail/?nettype=stagenet&onion=true )