The thing is, Apple still mentions both Sierra versions. In fact they in their typical Apple non-threatening wording (the velvet glove hiding the iron fist) say "if you need the previous version" it is available. It isn't the same as for Mavericks and Yosemite for which there is no mention unless you have purchased them in the past.
How to download macOS Sierra
https://support.apple.com/en-us/HT208202"If you have hardware or software that isn't compatible with macOS Mojave or macOS High Sierra, you might be able to upgrade to macOS Sierra."
and
"For the strongest security and latest features, find out whether you can upgrade to macOS Mojave, the latest version of the Mac operating system. If you still need macOS Sierra, use this App Store link: Get macOS Sierra. To download it, your Mac must be using macOS High Sierra or earlier."
How to upgrade to macOS High Sierra
https://support.apple.com/en-us/HT208969If you have hardware or software that isn't compatible with macOS Mojave, you might be able to upgrade to macOS High Sierra.
How about somebody "needing" one of the Sierras because even though they may have a computer that can run Mojave it breaks a lot of their software but they still want something higher than El Capitan?. That was not the case with the run from Mountain Lion to El Capitan. You had to and still have to download the EOL (El Capitan) unless you "bought" one of the earlier versions. Now they seem to be making a full upgrade to the latest and greatest optional (at least so the wording reads). The specs for Sierra and High Sierra read the same so if you can now still download one of the Sierras on a computer that can support Mojave that's a departure from past pattern. In fact of you can still download Sierra when all the same computers can run High Sierra that's also a big departure. So I guess the question is, can you really? I don't know. I run a 2008 iMac with Mavericks so right now the only option it shows is El Capitan.