It seems less a plea to stop supporting 'legacy versions' and more to stop support 5.2. Kind of. The initial part of that post is all about 5.2 and how only 0.64% of those on WordPress 4.9 (the latter is important as it goes up to around 2% if you include all versions).

He then doesn't recommend a specific version to move to but does talk about PHP 7 a lot. Moving to PHP 7 would, of course, prevent your plugin from working for about 90% of users.

So, I'm a bit confused. What is the recommendation for plugin developers here?