Hi, as most of the RNA sequencing analysis is focused on identifying DE miRNA expressed between comparison groups. How about non-DE miRNA is prospective study. 
Scenario 1. Suppose, I am investigating impact of Drug A for Cancer treatment. My hypothesis is that the expression of certain miRNAs should remain stable or not change after treatment or remain similar as before administering Drug A. How to find such miRNAs which did not change or induced by Drug A ? How can I do such analysis in DESEQ2 ?

See "Tests of log2 fold change above or below a threshold" in the vignette.

https://bioconductor.org/packages/release/bioc/vignettes/DESeq2/inst/doc/DESeq2.html#tests-of-log2-fold-change-above-or-below-a-threshold