The trains will operate on 26 July, but at a reduced frequency.
Currently, construction works on the trackbed do take place between Zurich HB and Zurich Binz. Therefore, the SZU line is completely closed from 14 July to 25 July, and from 26 August to 5 September 2023.
During the construction works, visitors of Mt. Uetliberg can take Tram 14 from Zurich HB to Triemli, and change there to the Uetlibergbahn. The transfer includes a 10-minute uphill walk between the tram and the railway station.
Fahrgäste S10 (szu.ch)
Btw, other good view points in Zurich, on the opposite hill of Mt. Uetliberg, are: Waid, and the panorama walk from Seilbahn Rigiblick to Zoo, tram station.