Exception #0 (Exception): Deprecated Functionality: DateTime::__construct(): Passing null to parameter #1 ($datetime) of type string is deprecated in .../vendor/magento/framework/Stdlib/DateTime/Timezone.php on line 301

From our dev partner: 

Having investigated it, we discovered that it's coming from Amasty Stock Status, and it occurs if a product's custom stock status includes the variable {expected_date}, which is not set (see attached). It is a bug in the Amasty module working with PHP 8.1.

We temporarily fixed it in vendor/amasty/stockstatus/Model/Stockstatus/Formatter.php but you'd better report it to the vendor.

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Just FYI