Yeah they run a fork of varnish 2 with inline C disabled, and a number of custom extensions (surrogate keys, tables/dictionaries, etc.) -- as well as likely customizations to support their massively multi-tenant deployment and more

Add in the necessity of porting vcl for all of their customers, and the prospect of upgrading varnish is obviously pretty daunting.

I suspect they'd rather focus on their new webassembly-based configuration solution, rather than try to keep up with changes in VCL