According to the FAQ post on synonyms and merging (current revision): "All good tag synonyms should eventually be merged (source)."
Having a tag synonym which is not merged causes various problems - the most important is probably the fact that the questions which have only the synonymized tag are missing in some searches.1
According to this SEDE query, there are 19 tag synonyms on this site where both the synonymized tag and the master tag contains some questions. (Below I have included the list of the tag synonyms returned by the query.)
Are among them some that are problematic and need some clean-up before merging? Or perhaps are there some synonyms which should be canceled?
For the tag synonyms that are safe, I would suggest that merging them might be a reasonable action. (I am fully aware of the fact that merging is not reversible - so it should be used only in cases where it is sure that the synonym is going to stay.)
Here is a link to the merging tool - mentioned by a per-site moderator in another discussion.
Here is the list of the tag synonyms returned by the linked SEDE query:
- newsletter → community-digests
- markdown-rendering → markdown
- code → code-formatting
- share-this → sharing
- latex → mathjax
- election-page → election
- downtime → maintenance
- date-format → dates
- lost-reputation → reputation
- reputation-page → reputation-history
- favorites-tab → bookmarks
- custom-question-lists → custom-filters
- exploit → security
- canonical-questions → canonical
- total-reputation → reputation
- careers-germany → legacy-stack-overflow-jobs
- site-specific-formatting → site-settings
- hot-tab → hot-questions
- careers-city-pages → legacy-stack-overflow-jobs
1The problems with searching are described, for example, here: Tag synonyms and searching. This post on Meta Stack Overflow is a result of the same issue: Converted tag synonym don't allow to filtering on converted tagged threads. Recently I made a related post on Mathematics Meta - the post contains an example of this, too. This post mentions also how this influence badges and dupehammer - but that is definitely less important than searching: What are differences between synonyms with and without merging from the practical point of view?
