The old function accepted an array or a string and the two behaviours have been separated, so a search and replace is wrong in one direction or the other.
// arrays
$this->assertContains($order, $collection);
// strings — and it is CASE SENSITIVE now, which it was not
$this->assertStringContainsString('Order', $html);
$this->assertStringContainsStringIgnoringCase('order', $html);The case sensitivity change is the interesting one: a test that passed matching Order against order now fails, and that test was asserting something looser than its author intended. Fixing it properly means deciding which was meant rather than reaching for the ignoring-case variant. Rector cannot make that call, which is why this is the part of the upgrade that needs reading.