What's the highest frequency that's been imaged by a radio telescope?
Some extensive search e.g. with scholar.google.com led me to a manuscript from October 2020 entitled Design and Characterization of 275-500 GHz Corrugated Horns and Optics for a Wideband Radio Astronomy Receiver which already has all the information in the title: It looks like there is current development towards another 500 GHz receiver.
The question mentioned a 0.95 THz receiver which is even above that.
I also found an online course at nrao.edu which argues that 1 THz is the absolute upper boundary to far infrared in astronomy.