Is there such a thing as a mandatory update in crypto? If a mandatory update exists, then you couldn't really call it a peer-to-peer network with a straight face.

This update brought stability improvements, optimisations and various improvements to the already extant Core wallet.

Far from it being simply a change in version delineation, you can read in depth about the (significant) improvements made here: https://stratisplatform.com/2019/03/13/stratis-full-node-3-0-2/