As good an overture as any to raise awareness for the nature and purpose of HTTP status codes in the 5xx class, in general, and 503, in particular.
Since 5xx codes are server responses, it is quite likely that the party running same also knows when they happen, so reporting them is probably redundant. Particularly, 503 designates a temporary issue by its very definition. 
I, for one, have been seeing a fair number of those (and adjacent ones, like 502 and 504) across different parts of Canonical’s infrastructure I come in contact with on a daily basis, e.g. packages.ubuntu.com, documentation.ubuntu.com — even the Discourse issues are probably also just 5xx errors, only not immediately obvious at times; I think it’s safe to say that the whole 2nd level domain ubuntu.com is affected one way or another. Since there hasn’t been an update on the DDoS in a while, we should assume that it’s still ongoing. To make matters worse, there’s also the “Ai” scraperbot scourge.