As per the migration guide
https://dev.twitch.tv/docs/v5/guides/migration/#default-api-version
The Preferred method is to use a HEADER. NOT a query string argument
v3 is the default API version and developers will need to set an Accept header to specify that they want to reach v5 endpoints. Developers can proactively include the v5 Accept header in order to properly test their updates. Below is an example and you can visit the Requests documentation for more information.
The Query String argument is undocumented and can change or break at any time.