Literary term for when the author purposely doesn't describe something so that the audience is left to imagine it

I think the term ellipsis may fit the bill here. While it frequently means the act of leaving out single words or parts of sentences, it can also be used to describe the choice to leave out entire scenes.

From the linked article, describing an excerpt from The Great Gatsby:

This is an extremely significant use of ellipsis from F. Scott Fitzgerald’s masterpiece, The Great Gatsby. In this excerpt, the narrator Nick Carraway has left a party with another man, Mr. McKee. They agree to go to lunch as they are in the elevator, and Fitzgerald then separates that with an ellipsis and suddenly Nick is standing next to this man’s bed, while Mr. McKee is in his underwear. The leap is surprising, and seems to omit not redundant information, but instead a very key moment in the relationship between these two men. Many scholars have taken this incident, made implicit through the use of the ellipsis, that Nick Carraway is, in fact, gay.

https://literaryterms.net/aposiopesis/

Aposiopesis is when a sentence is purposefully left incomplete or cut off. It’s caused by an inability or unwillingness to continue speaking. This allows the ending to be filled in by the listener’s imagination.

IV. Examples of Aposiopesis in Literature Aposiopesis is used in literature for dramatic effects. It can show that a character is overwhelmed with emotion. Or, it can allow the reader to fill in horrors or threats with their own imaginations. When characters pause due to strong emotion or searching for words, they appear more realistic and believable.

Example 1 An example of this may be found in Shakespeare’s King Lear. Lear is so upset he cannot think of proper punishment for his misbehaving daughters:

I will have revenges on you both

That all the world shall-- I will do such things--

What they are yet, I know not; but they shall be

The terrors of the earth!