The original is from phpBB's own template engine, the new formatting is from Twig.
https://twig.symfony.com/
I do not know the technical advantages such as performance. One huge advantage is someone who has expertise in HTML will probably already be familiar with Twig. It's utilized by multiple other web applications, Drupal for example.
I would imagine at some point support for phpBB's template engine will be dropped. That is personal comment and not from phpBB.
“Results! Why, man, I have gotten a lot of results! I have found several thousand things that won’t work.”
Attributed - Thomas Edison