I get a the error void-function autoload-rubric when calling
(el-get 'sync 'my-package-variable)
It appears the function el-get-ensure-byte-compilable-autoload-file in el-get-autoloading.el calls (the now) non-existent function autoload-rubric.
Maybe this function went away with the new loaddefs changes, and maybe that happened in Emacs 29.1?