bash/compatibility-level

o The `unset' builtin will unset the array a given an argument like `a[@]'.
  Bash-5.2 will unset an element with key `@' (associative arrays) or remove
  all the elements without unsetting the array (indexed arrays);

o arithmetic commands ( ((...)) ) and the expressions in an arithmetic for
  statement can be expanded more than once;

o expressions used as arguments to arithmetic operators in the [[ conditional
  command can be expanded more than once;

o the expressions in substring parameter brace expansion can be
  expanded more than once;

o the expressions in the $(( ... )) word expansion can be expanded
  more than once;

o arithmetic expressions used as indexed array subscripts can be
  expanded more than once;

o `test -v', when given an argument of A[@], where A is an existing
  associative array, will return true if the array has any set elements.
  Bash-5.2 will look for a key named `@';

o the ${param[:]=value} word expansion will return VALUE, before any
  variable-specific transformations have been performed (e.g., converting
  to lowercase). Bash-5.2 will return the final value assigned to the
  variable.