将引用变量推入结果数组，这样就不需要维护/增加计数器变量和切换布尔变量。
下面的代码只会在遇到STOPSALES值时销毁引用。对于所有其他值，它将创建并推送一个新引用，或者只是将新值推送到现有引用中。
代码：（Demo）

$result = [];
foreach ($array as $v) {
    if ($v === 'STOPSALES') {
        unset($ref); // no longer used (if it even existed)
        continue; // nothing more to do; go to next iteration
    }
    if (!isset($ref)) {
        $ref = [$v]; // declare/populate new reference with current value
        $result[] = &$ref; // push new reference into result at next available index
    } else {
        $ref[] = $v; // add current value to current reference/group because is a consecutive date value
    }
}
var_export($result);

由于处理的是索引输入数组，因此可以有条件地检查值是否为上一个索引，并在推送当前值时转换结果数组组的第一级键。
代码：（Demo）

$result = [];
$k = 0;
foreach ($array as $i => $v) {
    if ($v !== 'STOPSALES') {
        if (($array[$i - 1] ?? null) === 'STOPSALES') {
            ++$k;
        }
        $result[$k][] = $v;
    }
}
var_export($result);

下面是实现你的愿望的一种方法：

$array = [
    "2023-03-10",
    "2023-03-11",
    "2023-03-12",
    "2023-03-13",
    "2023-03-14",
    "2023-03-15",
    "STOPSALES",
    "STOPSALES",
    "STOPSALES",
    "STOPSALES",
    "STOPSALES",
    "STOPSALES",
    "STOPSALES",
    "STOPSALES",
    "STOPSALES",
    "STOPSALES",
    "STOPSALES",
    "2023-03-27",
    "2023-03-28",
    "2023-03-29",
    "2023-03-30",
    "2023-03-31",
    "2023-04-01",
    "2023-04-02",
    "2023-04-03",
];
$result = []; // array of subarrays
$i = 0; // incremented after a "STOPSALES"
$wasStopSales = FALSE; // to avoid creating multiple empty arrays with consecutive "STOPSALES"
foreach ($array as $element) {
    if ($element == 'STOPSALES') {
        if (!$wasStopSales) { // if that's the first consecutive "STOPSALES"
            ++$i; // let's move to another subarray
        }
        $wasStopSales = TRUE;
        continue;
    }
    $result[$i][] = $element; // add the current element to the current subarray
    $wasStopSales = FALSE; // last value wasn't a "STOPSALES"
}
var_dump($result);

结果是：

array (size=2)
  0 => 
    array (size=6)
      0 => string '2023-03-10' (length=10)
      1 => string '2023-03-11' (length=10)
      2 => string '2023-03-12' (length=10)
      3 => string '2023-03-13' (length=10)
      4 => string '2023-03-14' (length=10)
      5 => string '2023-03-15' (length=10)
  1 => 
    array (size=8)
      0 => string '2023-03-27' (length=10)
      1 => string '2023-03-28' (length=10)
      2 => string '2023-03-29' (length=10)
      3 => string '2023-03-30' (length=10)
      4 => string '2023-03-31' (length=10)
      5 => string '2023-04-01' (length=10)
      6 => string '2023-04-02' (length=10)
      7 => string '2023-04-03' (length=10)

如果你需要更多的细节，尽管问我。