The article that likely finally led to reddit taking action against r/the_donald:

https://thenextweb.com/opinion/2019/06/25/you-cant-offer-to-murder-cops-on-reddit-unless-youre-on-r-thedonald/

All it took was them threatening to kill cops instead of PoC, women, LGBTQ people, etc.

They also threatened to kill Trump's own DHS secretary because of a rumor that he saved some immigrants:

https://old.reddit.com/r/AgainstHateSubreddits/comments/c56us8/the_donald_threatens_to_kill_trumps_own_dhs/

More negative press that likely contributed to the move:

https://www.mediamatters.org/research/2019/06/24/A-pro-Trump-subreddit-is-full-of-calls-for-violence-in-support-of-Oregon-Republicans/224018

But it's important to note the quarantine of r/the_donald coincides with quarantined subs suddenly becoming accessible on mobile. They changed their quarantine system to ensure the quarantine of their biggest fascist sub doesn't actually affect anything.

All this really results in is an empty gesture; a one-time warning message ("This sub is quarantined. Are you sure you want to continue?") that can easily be ignored, while allowing the fascists who keep instigating deadly violence against minorities to continue operating the biggest fascist propaganda outfit in the world using reddit's infrastructure.

This 'action' is just reddit's management finding a way to continue to enable white supremacy in the wake of renewed media outcry.

Here's the admin's announcement:

https://www.reddit.com/r/The_Donald/comments/c5setv/quarantine/