Extension doesn't work on heroku-18

[matiosfree]

Hey all. Have a problem on heroku-18

1. Normal heroku-18 setup
2. heroku config:set HEROKU_PHP_PLATFORM_REPOSITORIES="https://heroku-v8js.s3.amazonaws.com/dist-heroku-16-stable/packages.json"
3. added "ext-v8js": "*" to composer.json -> require
4. Procfile 'web: vendor/bin/heroku-php-apache2 web/'
5. Ran "composer update" on heroku and got: "The request PHP extension ext-v8js * is missing from your system. Install or enable PHP's v8js extension."

PS https://heroku-v8js.s3.amazonaws.com/dist-heroku-18-stable/packages.json is missing and I tried to use dist-heroku-16-stable