Crop a video to 2:3, the vertical 35mm frame. It is 3:2 stood on end, and it matches a 4x6 print in portrait.
Narrower than a feed post
2:3 sits between 4:5 and 9:16. Against a 1920 px height it is 1280 px wide, where 4:5 would be 1536 px and 9:16 would be 1080 px.
That makes it a poor fit for a social feed, which expects 4:5 or 9:16, and a good fit for print-derived layouts and portrait photo grids where the surrounding stills are already 2:3.
The crop is severe from landscape
A 1920x1080 frame cropped to 2:3 keeps 720x1080: 37.5% of the width. That is close to the 9:16 cut and carries the same risk of losing anything composed away from the centre.
Fit with bars keeps everything instead. The full picture lands 1280x720 across the middle of a 1280x1920 frame with 600 px of background above and below.
From vertical footage
A 1080x1920 phone clip cropped to 2:3 keeps 1080x1620 and trims 300 px of height. That is a gentle cut, and it is the usual reason to be at this ratio: taking a phone recording into a layout built for portrait stills.