Has this feature disappeared? Or do I need more reputation to suggest a tag synonym?
Click on the tag you want to suggest a synonym for. There is link to right of top users and new answers, called 'synonym'. Click on that. Example:
The requirements for tag synonyms are listed on each synonym page and are:
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 of 5 or more, 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.