Custom Fields - Where Do They Go?

[PaulM1]

In order to re-use Custom Fields, I understand the user either needs to:

1. Save as a Look
2. Save to a Dashboard
3. Copy the URL (with the QID)

#3 is what brings me to this question. If Custom Fields can be re-accessed by navigating back to the URL with the specific QID, then they are never fully deleted from the Looker environment. Correct?

And if they are never fully deleted from the environment, what does that mean when you have users adding dozens (or even hundreds) of custom fields over time, and never using them again?