hello, I'd like to know how to set all users subscribed to Digest
I also noticed that when I flag "follow_topics_you_create" and "follow_topics_you_reply_to" those options are not saved (so I can't enable them)
Setting all users isn't possible. There's a button in the ACP to disable them all, but not enable them. (But as they're disabled by default, it doesn't really explain the button in the ACP, an on /off switch in the ACP would be better imotbh) You would have to go through and turn them on in then users settings, or encourage users to do so.
The two options do follow and create are fixed in the latest master.