I'd like to report that the first link in the UPDATING file in the src tree seems broken. Here's the broken link: https://www.freebsd.org/doc/en_US.ISO8859-1/books/handbook/updating-src.html.
Created attachment 186205 [details] base/{README*,UPDATING} URL fix The same link appears in README and README.md (also in base). The attached patch corrects all 3 occurrences.
The UPDATING file seems to be up-to-date now, which makes this issue no longer relevant.