Monolog's first commit was on February 17th, 2011. That is almost 5 years ago! I have now been thinking for quite a while that it would be nice to start on a v2, and being able to drop some baggage.
One of the main questions when doing a major release is which minimum PHP version to support going forward. Last summer I decided I wanted to do a big jump from 5.3 and directly target PHP 7. It provides a lot of nice features as well as performance improvements, and as Monolog is one of the most installed packages on Packagist I wanted to help nudge everyone towards PHP 7.
Now that PHP 7 has been released, he's moving even more towards this goal for version 2 of the popular tool. He talks about "the road forward" and links to a milestone that's been set up with issues to correct and features to update before v2.0 can be called stable. He does offer a word of warning too - if you use
dev-master for your Composer installs, update it to use
^1.17 instead as the main branch will break soon with the work for v2.