Shield Mode: First-Time Chatter Constraint should operate through Suspicious Users (aka Low Trust Users)
Shield Mode allows for messages from first-time chatters to be silently dropped.
A better implementation of this restriction would operate through low-trust-users (where the chatter is marked as restricted in the suspicious users interface), so mods can see the messages that were trying to be sent, and optionally unrestrict good faith first-time chatters.
With the current implementation, mods don't know who are the blocked first-time chatters or what message content they are trying to send. Via the suspicious users interface, mods could unrestrict first-time chatters that appear in good faith while shield mode is activated.

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Durss commented
For what it's worth, allowing to filter out those messages for the broadcaster is, to me, a good idea.
Example:
For the last few weeks in france someone is hate raiding looots of streamer girls with messages that are actually horrible to read (many of them will attempt to pursue him legally as his identity is known but in the meanitime, he's still terrifiying streamers).
Preserving them from reading these messages is, I think, a good thing if not the most important thing to do. Seing some of them faces turning horrified live tells everything...For now, no twitch tool allows this.
Maybe allowing to switch low-trust messages, and these first-time chattern, to mod-only for approval would answer this problem.? -
KVariable commented
Op knows some stuff about moderation. Allegedly.