升級 Apache2 套件看到的訊息：
apache2 (2.4.25-3+deb9u5) stretch; urgency=medium
* This package upgrades mod_http2 to the version from apache2 2.4.33. This fixes a lot of bugs and some security issues, but it also removes the support for using HTTP/2 when running with mpm_prefork. HTTP/2 support is only provided when running with mpm_event or mpm_worker.
-- Stefan Fritsch <sf@debian.org> Sat, 02 Jun 2018 09:51:46 +0200
2.4.33 之後，mod_http2 不支援 mpm_prefork，只有支援 mpm_event 和 mpm_worker，詳細內容可見：