Let's collect knowledge like we did with iOS 11, iOS 10, iOS 9, iOS 8 and iOS 7 – paste in your notes and share what you've learned, and somebody else will organize it later. :) If you want to ask questions and share tips over chat with other developers, see How to use IRC for how to connect to #theos and #iphonedev.
What has changed in iOS 12? (Classes, frameworks, etc.)
OpenSSH is not available through the normal means. To install OpenSSH enable the option to install OpenSSH on the next jailbreak in unc0ver's settings.
Apps compiled and installed to /Applications aren't run as root. Unc0ver's version of Cydia modifies binaries' entitlements to include the following if they don't already:
<key>platform-application</key> <true/> <key>com.apple.private.skip-library-validation</key> <true/> <key>com.apple.private.security.no-container</key> <true/>