+1 on the proposal, with an extension flag.
I think we should continue to be strict about language extension flags. Extension flags allow the programmer to signal very clearly which features they're using, which allows us to write code that will be correctly rejected with a useful error message both by older versions of GHC and by hypothetical other compilers that don't support the extension.
If we start to relax the policy of having extension flags, then it's hard to know where to stop. Yes we have already diverged from Haskell2010, but so far there was a compelling reason to do so in each case: e.g. we had no good way to support both versions of the Monad hierarchy, and for NondecreasingIndentation I think we had been accepting the extended syntax (without the flag) already in previous versions.
Cheers
Simon