Hide "This iPhone is managed by your organization" message from the lock screen
Removing the management profile

Someone else mentioned how to remove the management profile from the phone. That has some potential issues:


Your boss may not want you to remove the profile, which may lead to undesired consequences like being fired
You may lose access to any resources the profile allows you to access
You may start sending things unprotected directly over the Internet rather than through the VPN your profile sets up for you


Why the message probably can't be removed

As far as leaving the profile there, it's very likely that you can't remove the message because a lot of people are used to their personal iPhones being secure and private and they need to be informed when their privacy may be at stake because they may be being watched.

If they're issued a work phone, they may incorrectly assume that the security and privacy are just as good as their own iPhone.

If they let an untrustworthy acquaintance use their personal iPhone, they may have no idea that it's even possible for the acquaintance to take a few minutes to begin managing the phone and to then spy on them later.

Imagine if the boss or the untrustworthy acquaintance could just turn off the warning right after beginning to manage the phone. The user of the phone would then be kept in the dark about potential spying.

There's no easy way for an iPhone to distinguish between a spy disabling the message and the user disabling the message, so there's no easy way to allow for what you want while still ensuring users receive that vital information.