They're Rona'd. It's just one of the things being impacted by the current crazy supply chain challenges. They aren't super necessary, but do provide a backup method of monitoring.
My float valve failed when I went on vacation, day 1 of a 15 day trip and when I was already about 1000 miles away by car. I recoded my ATO to run based on my salinity probe at just a hair higher salinity than was normal just to decrease the likelihood of a flood. When I got home, my water level was about 1" lower than normal and everything was fine. It turns out that I knocked my cable loose because of my last minute idea to run RO tube to the top of my basement steps so that my house sitter could easily refill my ATO.