Call to deprecated function db_drop_index(). Deprecated in drupal:8.0.0 and is removed from drupal:9.0.0. Instead, get a database connection injected into your service from the container, get its schema driver, and call dropIndex() on it. For example,

Total occurrences Projects affected Topmost group Category
10 6 10000 Not covered by rector
Filter projects
Project Group Occurrences Sort ascending
user_import 1.x-dev 10000 3
drupalchat 1.x-dev 10000 2
testswarm 1.x-dev 10000 2
nodeorder 1.x-dev 10000 1
skinr 2.x-dev 10000 1
multiplechoice 1.x-dev 10000 1