Rust is doing the same thing (although on the level of crates (packages) instead of individual files). When you want to use new keywords like async/await, you have to declare your crate as using the 2018 edition of Rust (as opposed to the original 2015 edition). Crates using different editions can be freely mixed, so when a new edition is released, everyone can migrate to the new edition at their own pace.
Neither of these adds new keywords (ReasonML switches some around, e.g. match is now switch) but maybe one of these approaches might work for language features or breaking syntax changes or revisions.
Will you need a new keyword to enable your option for new keywords?
Having said that, we can probably implement it using only existing keywords. Maybe: import java.lang.extensions.assert.
(I assume Java has some reserved part of the package namespace they can use for this purpose. Even if they don't, it shouldn't be difficult to find a package name that hasn't been used by anyone yet)
The only language I can think of that does this extensively is Haskell (and some flavors of regular expressions). The main downside I have noticed is that the ability to do this allows for the language to add a bunch of complexity that would otherwise be prevented by the need to preserve backwards compatibility [1]; but, if the language writers can remain disciplined, it seems like an effective way to grow a language without the growing pains the article is discussing.
For adding keywords, this approach doesn't even add that much maintenance cost, as all the additional complexity is confined to the lexer.
[1] Or, as has happened several times with Haskell, add features that too broken to even be considered adding but for the fact they are always behind a flag.