While the more options there are to modify quantity and quality of visual objects the better - games based on the unreal engine are generally very good at that - current issues shouldn’t surface at all on high end hardware.
So even if the client were already optimized for gameplay - which it most likely is not ( debugging code, limited concurrency, verbose logging, etc.) - it doesn’t really matter how many options they provide to deactivate visuals, the cause would still be badly written software.
Instead, optimizing the server to manage the network delays of more than 100 players battling in real-time in a single spot should pose the primary challenge.
One way to deal with this issue is constant mass-testing and debugging of bursts of as many actions, inputs, transactions as possible.
Dropping a boss in town would be an example of such a stress test which also covers graphic performance. However, for now they probably focus less on final client performance as long as it is set to produce verbose and replicable information.