I think my issue is identical to #25834 but I thought that was resolved. Was it, or is that still forthcoming?

In my dev environment with all error reporting on I receive

"Deprecated: Array and string offset access syntax with curly braces is deprecated in \libraries\vendor\joomla\string\src\phputf8\utils\unicode.php on line 46
Error: Failed to start application: Failed to start the session"

Code at line 46 is: $in = ord($str{$i});

Actual result

System information (as much as possible)

Joomla 3.9.20 (an upgrade not fresh install)
PHP 7.4.8
WAMP on Windows machine

Additional comments

Thank you.