The desktop client logs and sends lists of currently running processes by default, and they also collect usage data (which channels you open, how long for, who you’re interacting with). In the settings, there’s literally an option for “Use data to customize my Discord experience”. And sure, they don’t show ads, but their third-party integrations do. Article with sources
In the end, processing and storing millions of texts, images, videos and files permanently, and hosting all those live voice and video calls, and making updates to the clients, will always cost more than what they get from Nitro and server boosting. Discord isn’t profitable; they have to make the deficit up to shareholders somehow.
At least you can block some of the telemetry with uBlock or similar