Aliasing single → solo
Link to alias

Reason:

seems to be the intention behind the tag

EDIT: The tag alias single -> solo (forum #272261) has been approved by @Millcore.

+1
There's no common single_* tags either, so that's unlikely to get tagged in cases where the users forget the underscore.

+1. It might be a tag from another booru, but the intent is more clear than in most other cases.

The only counterpoint I can think of is in regards to the Couples tag, perhaps on a valentines post, showing that there's a person that is "single" as in without someone, while around them are other people in pairs.
solo_focus may be needed in that case

The tag alias single -> solo (forum #272261) has been approved by @Millcore.