An example of curly brace correction is available in another project's commit.
A migration script to automatically make these changes is here.
This is an issue since 2008 when both square and curly brackets were used interchangeably for arrays and string elements though it was the intention to sue them for separate purposes as explained at:
https://wiki.php.net/rfc/deprecate_curly_braces_array_access
Though currently in PHP 7.4, only the square braces can be used to initialise arrays and also assign to them values, the curly braces can generally be used everywhere else where the square bracket is used.
Other threads elsewhere that refer to this problem are at:
https://github.com/squizlabs/PHP_CodeSniffer/issues/2558
https://stackoverflow.com/questions/59158548/array-and-string-offset-access-syntax-with-curly-braces-is-deprecated