We skip forward proxies in backend-focused system designs because system design interviews focus on:

• Backend architecture
• Scalability, consistency, fault tolerance
• Microservices, queues, storage

So we mostly diagram server-side architecture, and clients are treated as a black box.

However, API Gateway (and sometimes even the LB) is acting as a reverse proxy and we need it all the time in system design interviews. Just put a load balancer before it and put multiple instances of API gateway.