How should I use decorators in transpilers today?
Unfortunately, we're in the classic trap of, "The old thing is deprecated, and the new thing is not ready yet!" For now, best to keep using the old thing.
The decorators champion group would recommend continuing to use Babel "legacy" decorators or TypeScript "experimental" decorators. If you're using decorators today, you're probably already using one of these versions. Note that these decorators depend on "[[Set]] semantics" for field declarations (in Babel, loose mode). We recommend that these tools maintain support for [[Set]] semantics alongside legacy decorators, until it's possible to transition to the decorators of this proposal.
Babel 7 supports the decorators proposal presented to TC39 in the November 2018 TC39 meeting. It's fine to use these for experimental purposes, but they face significant performance issues, are not yet widely adopted; we don't plan to continue pushing for this proposal in TC39. As such, we recommend against using this version for serious work. In follow-on proposals to add more built-in decorators, we hope to be able to recover the extra functionality that the November 2018 decorators proposal supported.