>Was this applied retrospectively to existing customers

They are quite good at costs remaining predictable. However, a few years back they cut the low-end hosts 1Gbps unlimited data transfer down to a 20GiB/month cap, and wanted everyone to go full cloud/retard to fully leverage the hardware infrastructure.

If you serve large files, a CDN may have a very narrow use case where the budgets make sense. If you are already pushing 23 TiB/month, than cloud providers are usually not worth the effort. Some rent colocation rack space. =3