Explanation
Posting in any other ratio means the platform either crops your video or pillarboxes it, and both outcomes cost you. Cropping removes whatever was at the edges, which is frequently the product or the on-screen text. Pillarboxing announces that the video was made for a different platform, which is a reason to scroll before the hook has even played.
The safe habit is to frame for the ratio from the start rather than convert into it, and to keep anything essential away from the top and bottom eighths where captions, usernames and interface buttons sit. A perfectly framed subject is no use if the platform's own UI is on top of it.
Because all three major surfaces share the ratio, one correctly framed file is genuinely portable, which is what makes cross-posting worth doing rather than a compromise.
The common mistake
Putting your caption or logo in the bottom eighth of the frame, where the platform's own interface will cover it on every surface.