Hello imfaisal87,
"Question: Is it worth to certify a 2017 version in 2019 or should I wait for a 2019 edition (not sure if it will come or when)"
In my opinion this exam is well worth studying for if you want to get a better grasp of the fundamentals and many specifics of vSAN - there will likely be a delta for any new exam anyway. From a user/administrator perspective, most of the fundamentals concepts and mechanics are fairly agnostic for all of 6.x other than core changes like Unicast/Multicast, so whatever you learn studying for this exam will make any future exam easier.
"It's been a while for me, but as far as I remember that most of the questions were about design, best practices, and configuration maximums. So I think this certification is less suitable for operational use, because then the vSAN cluster is already in operation and doesn't change much. "
sk84, I feel that (where possible) any administrator of any environment should understand as much as they can about the products their workloads rely on - this helps make them more capable and efficient administrators. Thus I would tend to disagree that the content is only useful before deploying/designing vSAN as the blueprint covers a lot of topics:
It is of course entirely possible your exam had more questions from the design perspective than average, I recall mine (2 years ago) being quite varied - but then again it's not that long an exam so it is potentially more possible to be (or seem) skewed.
Bob