The opportunity status field has 5 values, but we wish to only use 3 of them. Is it possible to hide values from a drop-down field? I can't make the two that are not needed inactive as they are used elsewhere.

I would push back. That seems unnecessary and less than maintainable. It's disrupting native behavior in a way that will be confusing to even smart users. If you must, I believe this can be done with

Field.removeSelectOption()

.