The current Online Players window takes up space for things like "server info" and "player info". This information is mostly static.
Some players have indicated they would appreciate if this information could be hidden in some way, as they are not interested in that information after an initial look. During design of the Online Players window this also came up, and we decided at that point to postpone this feature till we have a bit more information how people use it / what they would like.
The tricky part about this request is, that 10 people have 11 opinions how to go about this. And it has the tendency to get out of hand real quick, with some strong-voiced opinions being thrown in the mix. We have to be careful to not design this for the 0.001%, but come up with something most players enjoy / can use.
Additionally, the GUI framework of OpenTTD is limited in the amount of visual elements it supports, other than manually drawing them (which we should with care, as maintaining those is a pita).
My suggestion is to come up with something simple, both in implementation (so reusing existing GUI elements) and in design, and make a proposal for that.
The default "just to be clear" note: despite me making this discussion doesn't mean I am interested in building this GUI ;) I merely relay information I received from various members of the community to a single place and suggest a way of working :)
To clarify, as I'm "some players", previously I liked to leave the window open and pinned while running multiplayer games, to keep track of who was on the server. See screenshot below. This isn't really convenient with the new window that takes up much more space with the server information etc.