If interbreeding within the hybrid zone of the mountain pass decreased over time, what would you expect to happen to the Western and Eastern deer populations?

The two population will become separate species due to allopatric speciation

Explanation:

Allopatric speciation is also known as geographic speciation. In this speciation two populations of the same species isolated geographically from each other which prevents gene flow between them and the two populations evolve separately and over time, it leads to the creation of a separate species.

So here as interbreeding within the hybrid zone of the mountain pass decreased over time so it will lead to speciation in these populations and they will evolve into two separate populations due to lack of gene flow as a result of geographical isolation.