How did I work around the «Invalid credentials (HTTP 403) for 'https://composer.amasty.com/community/packages.json', aborting» fatal error on any Composer `update` / `require` command?

How did I work around the «curl error 28 while downloading https://composer.amasty.com/community/packages.json: Recv failure: Connection was reset» fatal error on any Composer update / require command?