In very rare cases, someone complains about the removal of pattern lock, but it's a terrible feature and shouldn't be included in modern Android. It's a much worse variant of a PIN which gives people a false sense of security and encourages choosing an insecure lock method. We also didn't want to support it for our duress PIN/password feature or our upcoming random PIN/password generation feature. It's not really suited to either of those things. Pattern lock is genuinely an awful feature and shouldn't be included in modern Android, but they don't want to upset people by removing it. Extremely few people care that we remove it, and we've helped them by getting them to use a PIN instead especially if they use a proper random PIN.
> developer mode, adb, and sideloading
These things can be disabled with a device management app already, if what you mean by sideloading is installing apps via an APK or arbitrary app store. Sideloading an OS update to recovery has all of the usual signature verification and downgrade protection.