Cine lenses should work great for still images. If I ever went deep into portraits, it was something I would consider for a different look. I have been surprised to only see 1 or 2 photographers try this. Granted, the cine lenses are very large and very expensive.

My understanding: T stands for transmission. It is the actual measured light that makes it through the lens normalized to be equivalent to F-value. But it is real. F stops are calculated, being the focal length divided by the aperture diameter. But the values often rounded. For example, here is a recent patent on RF f/1.8 lens, most of the reported f values in the patent are actually 1.85. I've seen worse go by. So just like reported focal lengths are usually rounded up, reported F-values are usually rounded down. Photographers over the years have accepted this. However, cinematographers often have different lenses shooting the same scene or transition from one scene to the next and put a higher priority on exposure continuity, thus the industry responded given them T-stops where T-1.5 is the actual light coming through and T-1.5 is T-1.5 regardless of the lens. DXO actual measures the transmission of lenses, here is a recent write up on the RF 24-105. You can see it is actually a T 4.2-T-4.4 lens. Which is actually very good. I've seen lenses where the T-number is 2/3 stop different from the F-Number.