We launched a new search experience that will allow you use search terms and refine the results by content type (Q&A, blog posts, discussions, etc.) and depending on the content type, more filters will be displayed. For example, if you choose Q&A, then you can choose specific tags, languages and whether the questions have or no responses.
Please visit https://community.sap.com/search/ and let us know if this is what you were looking for.
No, it's not currently possible to filter search result by tag using interface.
Please read about possible workaround here: https://answers.sap.com/questions/12889100/how-to-limit-community-search-by-tag.html
This question was already asked number of times.