Without a key
These endpoints describe the platform: which venues and instruments are collected, what a query may ask for and within which limits, and whether a query you have written is valid. None of them returns a venue’s figure, so none of them needs a key — you can see what exists, and check your query against the real grammar, before deciding to buy an answer.
The same coverage is published as a page on debyko.com; this is the machine-readable form of it, and it is generated from the same registry the collectors work from, so the two cannot drift apart.
GET /v2/plans is the plans the same way: every plan sold from the site, its price and each limit, read
from the plan table the server enforces them from, so the numbers are the ones a key is held to.