models.php

$reallyOldUnreadMessage = factory(Message::class)->create([
    'created_at' => now()->subMinutes(37)
]);

$oldUnreadMessage = factory(Message::class)->create([
    'created_at' => now()->subMinutes(36)
]);

$recentMessage1 = factory(Message::class)->create([
    'created_at' => now()->subMinutes(20)
]);

$latestMessage = factory(Message::class)->create([
    'created_at' => now()->subMinutes(16)
]);

RecentUnreadMessages.php

// 1) If the user's most recent unread message is older than $minutes (15 is default)
// 2) Check if the previous message is unread and within $minutes
// 3) -> repeat 2) until no more unread messages match criteria

$unreadMessages = $channel->messages()
    ->where('created_at', '<', now()->subMinutes($minutes))
    ->whereNull('participants_read_status->' . $user->getUuid())
    ->latest()
    ->get();

$tooOld = false;

$messages = $unreadMessages->filter(function(Message $message, $index) use ($unreadMessages, $minutes, &$tooOld) {
    // Always return the first message as it is legit based on the initial query
    if ($index === 0) {
        return true;
    }

    // Exit if we have determined one message is more than 15 minutes older than the rest
    if ($tooOld) {
        return false;
    }

    // Check if the next closest message is within 15 minutes
    if ($unreadMessages[$index - 1]->created_at->diffInMinutes($message->created_at) < $minutes) {
        return $tooOld = true;
    }

    // Filter out the rest
    return false;
});

return $messages->values();

This is working but seems kind of loose.

The result should contain $latestMessage and $recentMessage1 while filtering out the older unread messages.