> With the AGPL license being legal kryptonite I wonder if license compatibility drove the decision

Unlikely. There's a number of other strong indications that basic maintenance was being neglected, including shell transcripts showing that at least one server was running FreeBSD 10.1 (released in 2014, end-of-life in 2018), and PHP code using the mysql extension (which was deprecated in PHP 5.6 = 2014 and removed in PHP 7.0 = 2015).

It's probably not a coincidence that 4chan was sold to a new owner in 2015.