The warning is triggered when at least 10% of the flags you've raised, processed in the past 7 days, have been declined. Depending on how many flags you've raised, this could be as few as one flag!
An 80%-90% accuracy rate really isn't that bad - it depends a lot on why that flag was declined. And the only way for you to find that out is to go review your flagging history.
This change was implemented to help folks learn what is and isn't worth flagging. While the most prolific flaggers have long been in the habit of reviewing their flag histories, most users do not - and so there is often a fair bit of confusion as to what (if anything) happens when a flag is raised. This warning attempts to close the loop: if declined flags are more than rare, you might want to spend a bit of time figuring out why - maybe your flags were perfectly appropriate when you raised them but the situation changed by the time they were handled... Or maybe you're flagging things that aren't really problematic.
In other words, treat this as an opportunity to learn something, not a condemnation.