I've found some older Drupal 7 info on this, but not much for Drupal 8+ yet. What I want to do:

• Give users the ability to set a 'notify me' flag on nodes
• When a comment is added to the flagged node, all users who have flagged it get an email notification

So far...

• I have the rule set up correctly to allow users in role x to set the flag. It has another action on it that is tested and working.
• I can select the 'Send email' action.
• I arrive a set of fields to specify, including the "To" field for an email address. It looks like I can select a flagging uid of users who set a flag.
• There doesn't appear to be any way to get the email address of a user who flagged into the "To" field.

Maybe this isn't yet possible, but am I perhaps missing something?