Gatsu it's just a warning for the developer of the software, in this case the extension visible in the path to the PHP file. You don't need to worry about it. You can let the developer know about it if they aren't already aware, but they don't really need to change anything until a new major PHP version gets released.
Note that some deprecation warnings (maybe not this one) are inevitable. Because Flarum 1.x supports PHP 7.3+, it's possible that sometimes the only way to implement a feature to work all PHP version from 7.3 to 8.3 will throw deprecation warnings for people on PHP 8.3.
