Usually moderators wait to see the community's reaction to such propositions. Two upvotes (as I write) doesn't show significant community support by Meta.SO standards.
Your proposition seems weird to me, but I don't know anything about Symfony. Is there really a need for separate tags for Symfony 2.1 and Symfony 2.0? And for Symfony 2 as distinct from Symfony 1? Version-specific tags should only be used for questions that only apply to this version. If a question applies to both Symfony 1 and Symfony 2, it should be tagged symfony only. Unless Symfony completely breaks compatibility with every version, symfony-2.0 should probably be merged into symfony2 and removed as a tag name. If there really needs to be a separate tag for 2.0 and 2.1, they should not be synonyms.