Does deactivating a device delete anything?

No. Nothing at all. Your trades, settings, goals and licence are on your account, not on the device — a device is only a permission to reach them.

Sign in again on that machine and it takes a slot again, assuming one is free, and everything is exactly as you left it.

The one thing that is device-specific is your AI provider key, which deliberately never leaves the browser you typed it in. That is not affected by deactivating either — but it is worth knowing that a browser whose data you clear loses the key and needs it pasted again. See where your key is stored.

Deactivated devices stay visible under Previously deactivated, greyed out. That is a record, not a problem.