Sir Lancebot already have issue about getting copy of bookmark: python-discord/sir-lancebot#341.

Thanks, edited my comment to remove the bookmark part.

Would removing a reaction opt-out of the reminder? If so, would people spam-toggling the reaction be a concern for the reminders endpoint of our API?

I hadn't considered any extra opt-out functionality besides canceling, but if we want to implement something like that, we might have to add a cooldown. Regardless, I don't think we should necessarily react to users when they opt-in/opt-out, so the only concern would be from a bot resource standpoint.

One way to combat that would be to not run all the extra checks of whether the user has permission or not, etc, but to just add/remove users from a list. The checks would still run for the initial message, but the reactions just interact with a list.

How would edits be handled? If a user opts in, and then the initial user deletes or modifies it, would it modify it for all users, or create a copy for themselves?

I don't see a reason to complicate this. If it's deleted or edited, we just send whatever we have at the time it's supposed to be delivered. If that means not sending a reminder, so be it.

True, that makes sense.

Is this issue approved? If so, I'd be interested in working on this.

I made a quick prototype using the UI interaction button (similar to .bm), which will not support opt-outs at all.

However, using reactions as originally proposed lets the original author see who also opted-in to be notified. With this approach we'd have to perhaps have it edit the original affirmation message to "and will mention N+1 other(s)".

Yes, you can start working on it.

We should probably be moving away from using reactions; the button is better UX. I don't think the ability to see who else added themselves is important; editing the message with the new count is sufficient, but I do not consider that a strict requirement.

We will have to keep in mind the limits on how many people can be mentioned (if it sends DMs instead then that does not matter, but I forget how this command works).

Yes, all mentions are pinged as message content together with the reminder embed sent publicly:

So the mention limit will just be the 2000 character limit, counting all mentions to be in the <@ID> form. On average, the character limit might be exceeded when >90 different people click on the button to get mentioned, if I'm not mistaken.