$ brew update -v
Checking if we need to fetch /usr/local/Homebrew…
Checking if we need to fetch /usr/local/Homebrew/Library/Taps/homebrew/homebrew-boneyard…
Checking if we need to fetch /usr/local/Homebrew/Library/Taps/homebrew/homebrew-cask…
Checking if we need to fetch /usr/local/Homebrew/Library/Taps/homebrew/homebrew-core…
Username for [redacted]: remote: Enumerating objects: 7, done.
remote: Counting objects: 100% (7/7), done.
remote: Compressing objects: 100% (3/3), done.
remote: Total 4 (delta 3), reused 1 (delta 1), pack-reused 0
Unpacking objects: 100% (4/4), done.
From [redacted] Homebrew/homebrew-cask
2efd588…dd90b41 master -> origin/master
Password for ‘https://github.com’:
remote: Repository not found.
fatal: Authentication failed for ‘https://github.com/Homebrew/homebrew-boneyard/’
Error: homebrew/homebrew-boneyard does not exist! Run ‘brew untap homebrew/homebrew-boneyard’
Branch master set up to track remote branch master from origin.
Switched to and reset branch ‘master’
Your branch is up-to-date with ‘origin/master’.
Switched to and reset branch ‘stable’
Current branch stable is up to date.
First, rewinding head to replay your work on top of it…
Fast-forwarded master to origin/master.
Updated 1 tap (homebrew/cask).
No changes to formulae.
I then decided to run
$ brew untap homebrew/homebrew-boneyard
and now brew update works again without prompts:-)