Crop a video to 4:3, the frame used by broadcast television before widescreen, by consumer camcorders into the 2000s, and by slide templates built on the older projector shape.
The retro look
4:3 is the strongest single signal that footage is old, which is why it gets used deliberately on modern clips. Cropping to 4:3 costs a quarter of a 16:9 frame’s width: 1920x1080 keeps 1440x1080.
If the goal is the look rather than the shape, the crop matters less than what goes with it. The ratio alone reads as a mistake unless the grade, grain, or frame rate agree with it.
Matching archival footage
Cutting new material against a 4:3 archive is the other common reason to be here. Two things have to line up: the ratio and the pixel shape.
Older standard-definition video used non-square pixels. A 720x480 NTSC file is 4:3 on screen even though its pixel grid is not. The frame here is measured in display pixels, after any pixel aspect ratio the file declares, so what you see in the preview is what the export contains.
Pillarboxing instead
Fit with bars keeps the whole widescreen picture: a 1920x1080 clip lands 1920x1080 in the middle of a 1920x1440 frame with 180 px of background above and below it.