We used to flag for the recipient with a time that had passed to turn the message red in their inbox, so they would notice it. Now, it looks like there's no way to draw attention to it in that way.
Although effective, I can tell you that that isn't appreciated by most recipients as they have no way to reply in time when you set a date/time in the past. This puts the recipient under pressure and stress and gives them a sense of failure as they didn't comply with the request. The proper way would be to use the High Importance option for that so that it will get marked with ❗ in their Inbox.
If you are using the flag for recipient, then set it to an appropriate time for the recipient. For instance, if it is really important, 4 hours after you expect the recipient to have read it (considering business hours).
Abuse of features is exactly why a feature may not get implemented anymore and this happens to be such a feature as it is a low usage feature and abused by several spammers as well. I highly recommend to use the Help-> Feedback form to request the feature (Flag for Recipient) to stay and motivate why to keep the scales in favor of users and productivity. Improvements to the Flags feature is in focus now so hopefully we'll see it on the Roadmap soon and with the option to flag for recipients as well.
FYI:
In cases of (internal) abuse of the feature, users tend to create Message Rules to unflag the item or adjust their Conditional Formatting rules so it won't get highlighted anymore. The same applies to senders who mark all of their email as High Importance. It's a "The Boy Who Cried Wolf" kind of thing ;-)