Super old clientID’s (off hand I don’t recall the cut off but I think it was may 2018 or 2019 where expiring was introduced) can generate tokens that currently last forever/have no expires, that is why you have an expires_at of 0.
The documentation for the validate endpoint omits the key, as for the life of the API, it’s a recent addition (expiring tokens). At time of writing old ClientID’s can still generate (what I call) “god user tokens”, that last forever/have no expires (and where for normal oAuth you still do get a refresh token).
At some point Twitch will kill these old tokens and the ability to generate “God Tokens”
Has the “request” to fix the documentation