>Using ESI? VCL? Signed URLs or auth? Any other custom functionality? Are you depending on your provider's bot management features which are "CONTACT FOR PRICE" with other providers?

I have no idea what two of those acronyms mean. None of this is part of what a CDN offers.

Yes if you use DDoS protection, or cloudfare’s ZeroTrust or embrace $X proprietary features then what I said no longer applies.

I strictly said DNS and CDN.

ESI = Edge Side Includes think Server Side Includes on a CDN technology as supported by Akamai and used by sites like Ikea to deliver a fast maintainable experience

VCL = Varnish Configuration Language i.e. how you configure your Fastly services

If you're just using a CDN as a proxy then there's no lock in but plenty of sites are using CDNs for much more than that