I recently made a fresh installation of nextcloud on my Raspberry Pi 2, using dietpi Stretch beta.
I installed nextcloud via the included dietpi-software tool, which automated several steps. Everything about nextcloud worked quite well, even that I just recognized, that the php-json module was not installed.
On current nextcloud admin manual it says, that json is necessary to run nextcloud, which seems to be not true. I did not recognize any issue/error inside nextcloud, so I am wondering for what json is actually necessary? 
The dietpi-image uses Raspbian Stretch Repos, with up to date Apache 2.4, PHP 7.0 and MariaDB 10.1. Nextcloud is of course on up-to-date 12.0.0.29.
€: Ah, php7.0-json was installed automatically as dependency of libapache2-mod-php7.0, just not the mata package php-json ;).