Another database, different issue:

[30-Jul-2023 22:06:38 UTC] PHP Fatal error: Uncaught mysqli_sql_exception: Specified key was too long; max key length is 1000 bytes in /Users/scott/sites/thatsoft_store/ConvertDb.php:736
Stack trace:
#0 /Users/scott/sites/thatsoft_store/ConvertDb.php(736): mysqli_query(Object(mysqli), 'CREATE INDEX `i...')
#1 /Users/scott/sites/thatsoft_store/ConvertDb.php(395): ConvertDb->doQuery('CREATE INDEX `i...')
#2 /Users/scott/sites/thatsoft_store/convert_database.php(282): ConvertDb->updateTableCollations('coupons', 'utf8mb4_unicode...')
#3 {main}
thrown in /Users/scott/sites/thatsoft_store/ConvertDb.php on line 736

Keys on coupons table are as per basic install in 1.5.8.

Running in PHP 8.1 + MySQL 5.7.39