Request lobby list with filters
Right now you just get a list of public lobbies, that means that it is possible to get lobbies you are not really interested in. So a solution for reducing output would be to specify search filters.
One solution and the easiest would be to just specify fields as an object and provide values for comparison.
Example from one of my projects : https://github.com/BigETI/ElectrodZMultiplayer/wiki#listlobbies
Another solution would be to use a language that has been designed for this kind of problem like for example GraphQL: https://graphql.org/code/#go
The idea is to do this in a simple way. Using something like GraphQL for one filtering endpoint would definitely be overkill.
https://github.com/scribble-rs/scribble.rs/wiki/API-Docs#v1lobby---get-request should be extended with a JSON body that contains a filter field with sub-fields for desired filters, such as public: bool and game_state: bool. This can easily be extended in the future.
This should get some unit tests though :)