I was using cron-job.org to keep my Vercel serverless functions warm. Today my site went down and the only thing I can think is that I am no longer allowed to use external cron job functions since they have introduced there own.
Can anybody verify this is the case, as I cannot find information regarding cronjobs from external sources on Vercel.
I don't believe that is the case. Vercel has stated that there are even alternative cron job providers which pair well with vercel see: