Question: Have deep-space spacecraft always use some form of spread-spectrum for data downlink?
note: I'm looking for some insight into why, and any possible exceptions, not just a "yes" or "no". Thanks!
I was telling someone that the use of spread-spectrum in one form or another has become almost universal in civilian wireless communication and signaling (cell phone voice and 3G/4G/5G, WiFi, Bluetooth, GPS, ZigBee, LoRa (e.g. Understanding the relationship between LoRa chips, chirps, symbols and bit) etc. and it is interesting to note that WiFi and its resistance to multi-path interference has its origins in a couple of radio astronomers from Australia's CSRIO!
In April 2009, 14 technology companies agreed to pay CSIRO \$1 billion for infringements on CSIRO patents. This led to Australia labeling Wi-Fi as an Australian invention, though this has been the subject of some controversy. CSIRO won a further \$220 million settlement for Wi-Fi patent-infringements in 2012 with global firms in the United States required to pay the CSIRO licensing rights estimated to be worth an additional \$1 billion in royalties. In 2016, the wireless local area network Test Bed was chosen as Australia's contribution to the exhibition A History of the World in 100 Objects held in the National Museum of Australia.
I believe that the Gold code used in GPS can trace part of its origins to NASA's deep-space communications as well, though I can't find a reference for that right now. Note also that the importance of the Gold code is for timing reconstruction using correlation in the same way that NASA used it for range/rate, as well as for signal/noise benefits; see this and this answer for example.
Spread spectrum has many advantages, and one of them is signal-to-noise, which can be understood in terms of the Shannon-Hartley Theorem (see Am I using Shannon-Hartley Theorem and thermal noise correctly here?), and I believe that Voyager's data downlink always employs a bandwidth wider than than its bits-per-second rate would require, but I'm not sure; see the last paragraph in this answer to "How to calculate data rate of Voyager 1?" for example.