Are there widely used systems that support that? It seems like at most one person would bolt together something to process and route those. I doubt it is worth the time to define the format, and even if we did it seems unlikely we'd consistently get the format right when sending.
I had considered using the + syntax for things, like grpc-io-announce+go@, but that doesn't work well for multiple languages. KISS. There are very few of these emails, so recipients just look at the subject line. We won't be sending enough emails to the list to get good at a complicated process.