A portrait image can give room to a standing person, a tall object, or a close detail that accompanies a short observation. The extra height is useful when it serves the subject. It is less useful when a wide scene has to lose the context that makes the photo interesting. Decide what the post needs to show before choosing the crop.
This page selects our 1080×1350, 4:5 export for Threads. It is a consistent working size rather than an official rule for every image. Different post contexts and attachment sets can be presented differently, so the actual draft preview matters more than an assumed universal shape.
Check hands, feet, faces, and meaningful objects near the source edges. A vertical crop can make a person larger but accidentally remove the action that explains the photograph. If several people belong in the story, retaining the full width may be the better choice. Use Custom when the original proportions communicate more clearly than a tight portrait.