The 7.3.5 client itself is a masterpiece of WoW patch design – it just lives in a legally and technically grey area. Download at your own risk, and never reuse account passwords from such clients.
If you are trying to connect to a private server that runs on Legion 7.3.5, you cannot use the standard Battle.net download. The official app will force you to update to the latest retail version, which is incompatible with private servers. Wow Legion 7.3 5 Client Download