I don’t think it’s likely, for two reasons:
- Arch’s repository already has
mesawith support for these decoders, even in their latest staging branch.
So it looks like Arch Linux will continue to support these decoders via mesa (by their very nature of being “as vanilla as possible” in regards to upstream.) CORRECTION: Looks like Arch is straying from upstream in this case. 
Upstream Mesa is disabling support by default.
The mesa package for Manjaro is intentionally compiled without such decoder support.
There are two versions of mesa in play: Arch and Manjaro
- Even if an AUR version is created (to guarantee continued support of these decoders) it would be redundant and possibly introduce breaks in the future if it’s not properly maintained, let alone tested to work with the official repository libraries, dependencies, etc.
Arch Linux users would vehemently oppose such a package in the AUR[1], while Manjaro users would just request their already existing package (in the Manjaro repository) to revert back to how it used to be with full decoder support.
[1] Such a mesa PKGUILD in the AUR would violate their policies. You cannot say it’s a “patched version with extra features” when in fact it’s the exact same version with the exact same features, but you want to submit it for the sake of convenience for Manjaro users.