See this link: https://forums.concretecms.org/t/v-9-documentation-supports-php-7-4-lts-therefore-v-8-incl-addons-support-and-forums-should-be-supported/7666 and this:
https://forums.concretecms.org/t/version-8-extensions-to-be-removed-from-the-marketplace/7355
Our former developer left the concrete platform because of this.