Not that I object to the sentiment of not forward porting legacy APIs to WebGL2, I do take issue with the sentiment in general. I understand why you think this is how things should be, but I think this is wrong.
As of January 19th 2017, support for WebGL1 extensions is still fragmentary.
Picking out extensions with less than 90% support where support levels by UAs that do implement them is substantially higher (and that aren't compression related)
All of these extensions are older than 3 years, and the reason they're not well supported is because some or other UA didn't implement them, oftentimes citing "concentrating on other things".
Chief among the reasons why other UAs didn't see a pressing need to implement these ubiquitously supportable extensions, is because other UAs didn't send a clear enough signal early enough that these are in everybodies interest to be implemented.
WebGL1 unfortunately is going to be continued to be used for many years for a variety of reasons. I believe that WebGL Stats tracking of WebGL2 will demonstrate this in the years to come.
UAs share a responsibility to make WebGL better in all regards, this includes:
Extending the maximum parameter space of all WebGL versions (Apple, I'm looking at you)
Improving the support of existing extensions
Implementing new extensions
Supporting WebGL2 (Microsoft, Apple, your move)
Saying "WebGL1 is in maintenance mode" is really just saying you're abandoning it. Abandoning WebGL1 effectively means abandoning WebGL, period. You condemn developers to live with a situation for many years, that could easily have been vastly improved if UAs properly assumed their responsibilities and vigorously competed with each other to to provide the better WebGL implementation (of any version) than their competition.
There will be a time when its appropriate to abandon WebGL1. That time will not be here for many years. And although you should indicate very clearly that WebGL2 is now the top of the competition, that does not mean you can slack off in every other regard.