> carl sagan called METI “deeply unwise and immature"
It's repeated ad nauseum online, but always verbatim, just those few words and never a full passage, and never with a citation. In other words, it has all the hallmarks of an apocryphal quote or misattribution.
The reason I'm suspicious is because Sagan contributed to the Aricebo message[1], which is literally sending such a radio signal, and the the Voyager disc[2], which is similar. He even wrote an entire sci-fi novel[3] about it.
He describes radio contact in generally positive and hopeful terms in his book Cosmos. He of course acknowledges the dangers of encountering a more technologically advanced civilization, but he goes out of his way to contrast the frightening example of the Aztecs with other more peaceful first encounters such as the Tlingit. He also argues that any significantly more advanced species that had survived millions of years would necessarily have achieved zero population growth and would likely be peaceful. You don't have to take my word for it, you can read his own words in the Encyclopedia Galactica chapter of his book on the Internet Archive[4].
So, if the quote you cited was true, it would represent a late-in-life and somewhat surprising change of heart from cautious optimism to "dark forest" style paranoia. Personally, I believe it's simply one of the many falsely attributes quotes floating around the Internet.
[1]: https://en.wikipedia.org/wiki/Arecibo_message
[2]: https://en.wikipedia.org/wiki/Voyager_Golden_Record
[3]: https://en.wikipedia.org/wiki/Contact_(novel)
[4]: https://archive.org/details/sagancosmos/page/n184/mode/1up
[0]: https://www.science20.com/brinstorming/meti_should_we_be_sho...