In the latest version of the 2021 timetable, it looks like the SBB are still operating the alternating Singen/Stuttgart trains next year, so I'm guessing they are planning for the IC2 not being available? Certainly the 2021 timetable announcement don't mention this route at all, suggesting no changes (vs announcements for stock changes on other routes, including e.g. Pendolinos for Zurich-Munich and Girunos to the south - although both of those were noteworthy due to journey time reductions).
Based on my back of the envelope calculations, the entire Zurich-Singen+Zurich-Stuttgart rotation does only require 5 trains, although one of those is operated as 8 to 9 carriages (the 17:37 to Singen) instead of 6 carriages. [That's based on looking at all the Zurich departures, and figuring out all the places they end up going - it's 3 hours for a Zurich-Singen-Zurich turn, and 7 hours for Zurich-Stuttgart-Zurich, which also means that a given train will alternate between going to Singen vs Stuttgart.]