These two tags are both in use for a mix of different kinds of code generation.

• 3,690 × code-generation
• 247 × codegen

codegen seems to be occasionally used in the specific jargon sense “what machine code a compiler chooses to generate”, and also often used for various tools which have “codegen” in their name, but other than that, the two tags seem to have the same kinds of questions. (And if either were removed, it's likely it'd be recreated.)

Therefore, I propose that codegen should be made a synonym of code-generation.