It's at the very bottom of the settings page

I will look into the undrain links on the settings page not working. I just tried and there indeed seems to be an issue with them.

Did you know you can now also undrain individual threads. There's one catch: you need to know the original URL of the thread. (you can find out by logging out and viewing Pinside as a guest, then locate the thread and copy the URL). When you paste that in the location bar to load the thread directly the thread will load normally. Now scroll all the way down to the bottom and find the "undrain this thread" button (with the garbage can icon).

Drain is clogged

I just tested this by draining a thread and then undraining it again, through the settings screen. All worked well on my end. Could you please try again - maybe the issue was cache-related?

And if you go to a drained thread directly (pasting the URL in the address bar), what does the drain button at the bottom of the page say? Does it say "Drain thread" or "Undrain thread"?