Under the hood, we're turning the POST requests into get requests, so that limitation shouldn't affect us.
As far as I know, underlying C@E behaves like the one used in a VCL service.
What happens when the query string gets too long?
The question that had to be asked :D
First of all, with the current query string size, you can already send quite big queries (8kb), which should suffice for many use-cases. The introspection query, already quite huge for reference is 1kb.
For the ones who need more - splitting the values into `Vary` headers is an option to further increase it.
At some point - around 50kb there just is a hard limit.
If you then want to send an even bigger query - we have persisted queries on our roadmap. Then you'd just send a hash.