Maybe I am wrong, but it seems it is possible to filter only by tags used in a specific notebook.
Select the notebook
Click in the searchbar
Add the filter for your previously selected notebook (should be shown directly under the search input field)
Other - for this notebook possible - filters are proposed
Only tags are shown which are set in this notebook
This approach only shows 9 of the tags in the selected notebook. If the number of tags in the selected notebook is greater than 9, it won't show them all.
[Partial Feature Released in Version 10.2] Restore the legacy Tag selector / Tag filter
in Evernote for iOS Requests (Versions 10.0 and above)
Posted
This approach only shows 9 of the tags in the selected notebook. If the number of tags in the selected notebook is greater than 9, it won't show them all.