I'm not actually sure if we should issue a deprecation in 2.99...2.99 doesn't support the 3.0 interface, right?  It looks like here you've backported part of the 3.0 interface (the `register` part) but that doesn't mean the formatter fully supports the 3.0 interface since in 2.x (including 2.99) the notifications are sent with different arguments.  In fact, if the formatter uses `register` it'll be a 3.0 formatter that will not work on 2.99, unless the formatter author specifically creates a version for RSpec 2.99 (which would be ridiculous to ask formatter authors to do).

In addition, the fact that the loaded formatter doesn't appear to be RSpec 3 ready doesn't actually mean it's not ready -- consider that gems like fivemat and fuubar conditionally define the formatter class based on detection of what rspec version is running...so a warning issued here could be totally wrong (and thus confusing).

Finally, we're telling users to "upgrade to 2.99, get warning free, then proceed to 3.0" -- but this deprecation doesn't fit in with that.  There's no way to act on this deprecation while on 2.99 and solve it so that it is no longer issued.  The `rspec-legacy_formatters` is intended for usage only in 3.0, right?