Reza Rad has a workaround for cases in which you want to filter a Power BI slicer:
The idea of this blog post came from a question that one of students in my Power BI course asked to me, and I’ve found this as a high demand in internet as well. So I’ve decided to write about it.
You might have too many items to show in a slicer. a slicer for customer name when you have 10,000 customers isn’t meaningful! You might be only interested in top 20 customers. Or you might want to pick few items to show in the slicer. With all other visual types (Such as Bar chart, Column chart, line chart….) you can simply define a visual level filter on the chart itself. Unfortunately this feature isn’t supported at the time of writing this post for Slicers. However the demand for this feature is already high! you can see the idea published here in Power BI user voice, so feel free to vote for such feature
As Reza notes, this might get resolved fairly soon. Until then, check out his solution.