On 9th August 2020, I gave the exam for Acquia Certified Back End Specialist for D8 and I cleared it.
I would like to share my preparation journey. First you should do is always go through there guidelines of the particular exam for me it was Back End Specialist for D8. You must understand the exam pattern, if you have given any other Acquia Certification before then you should be familiar with it. There will be scenario-based questions for which you should have well experience with Drupal 8 workings. I felt the only preparation you can do is to brush up your knowledge about the topics which are mentioned in the Study Guide. The Study guide also says to rank your skills as per the topics. Try that out too before jumping into preparation.
My first place of choice was Drupalize.Me. It has a very well divided topic as per the specific exam. Please remember it will just help you to brush up your knowledge. So take your time and go thoroughly with the important topics like Drupal core API, Migrate API,Testing in Drupal, Theme Integration, many more. See all the topics are important but you have to evaluate your self on which topic you are lagging and mark them as the most important ones.
Don't forget to check Drupal.org it has all the good stuff there. Like the coding standards documents. Cache API is also pretty important and it's well-described there. I found myself reading the doc. too. They are pretty informative, you will surely find something which you didn't know before. I learned that max-age doesn't work for anonymous users.
The pro tip from me will me just go for it. Start preparing, take your time, feel confident about your preparation. Get relaxed and read all the questions very carefully. It's important that you always keep one thing in mind i.e. "I want to learn something new while doing this process."