Following this page tutorial for creating a new button for 7.0 fix pack 89 didn't work.
after initial investigation we realize that indeed mixins are not available starting with alloyeditor version 2
If the user wants to use mixins he needs to upgrade to at least alloyeditor version 2.11.0,
which is included started with fix pack 90, see https://issues.liferay.com/browse/LPS-102881 related to this update
the error user was facing is related to the part of creating the button https://help.liferay.com/hc/en-us/articles/360017882692-Creating-and-Contributing-new-Buttons-to-AlloyEditor#creating-the-button