I find that tag synonyms are very hard to find in the first place, for instance it took me a couple passes to find where to go to recommend one. So how would I (for instance), surface to the people who would vote on it that there is a synonym out there to vote on?
I went through this same discovery issue with
string-methods and trying to get it approved as a synonym for
string. It can be challenging for some of the lesser used tags to do this because according to the help:
Users with more than 2500 reputation and a total answer score of 5 or more on the tag, can suggest tag synonyms. Users with a total answer score (total upvotes minus total downvotes) of 5 or more on the tag, can vote for tag synonyms. Suggestions will be automatically approved when they reach a score of 4, and automatically deleted when they reach a score of -2.
So, what this means is that you need to go to the tag info page by clicking on it where you'll see a screen like below.
Click on Users at the top right of the page (where the red arrow points to the red box) which will take you to a page showing the top users of that tag and their reputation as in the image below. The smaller number to the right is the number of questions they've answered. The larger number to the left is the net count of the total number of up and down VOTES they received on their answers for that tag.
In essence, you need to look at the users on this list for the tag in question that you want to create a synonym for and find a way to get them to vote. Not exactly easy when you have no way to PM them. About the best you can do is post a message here.