As confirmed by the community manager of CD Projekt RED Marcin Momot on the Polish software house forum, the demo of Cyberpunk 2077 shown behind closed doors during the E3 2018 in Los Angeles was running at a resolution of 1080p.
As you can see at this address, confirmation came from the community manager on the official CD Projekt RED forum. Recall that the E3 2018 demo of Cyberpunk 2077 was running on a high-end PC characterized by the following hardware configuration:
CPU: Intel i7-8700K @ 3.70Ghz
MB: Asus ROG Strix Z370-I Gaming
RAM: G.SKILL Ripjaws V, 2X16GB, 3000Mhz, CL15
GPU: NVIDIA GeForce GTX1080Ti
SSD: Samsung 960 PRO 512GB M.2 PCIe
PSU: Corsair SF600 600W
The fact that with a PC like this the demo ran only at 1080p, at a time when all the high-end devices aim at 4K (especially in view of a launch between next year and the next), it is quite significant. Momot did not specify what the frame rate was but the reports of those present speak of 30fps although not exactly fixed, and with some decline here and there.
Recall also that the editorial staff of Digital Foundry has expressed itself on the demo of Cyberpunk 2077, praising the level of detail and the technical caliber. According to the words of CD Projekt RED, moreover, the game will be optimized even on the console, without creating any significant gap with the PC edition.