Typically, the tags with no questions are automatically removed by a script which runs once a day, this process is called tag pruning.
However, there is a known bug which causes that if both tags in a tag synonym have zero question, they cannot be removed by this script. See this bug-report: Auto-delete orphaned tag synonyms. This post contains even the query to find such instances, at the moment it returns one such synonym on the main site and nothing on meta.
One can even check that when searching among tags, the tag geometric-cyclicity is displayed. You can even see the synonym geometric-cyclicity$\to$cyclicity in the list of tag synonyms - you'll need to go back to December 2014 when it was created. (I've added both screenshots below.)
A screenshot showing what I get when searching among the tags:
A screenshot from the list of the tag synonyms:
A related question - if somebody stumbles upon a tag synonym like this, what is the right thing to do? Is asking on meta the safest option? (Although I assume that things like this are rather rare.)