Description

Indexes are treated as destructive operation (element*removal) in https://github.com/magento/magento2/blob/2.3-develop/lib/internal/Magento/Framework/Setup/Declaration/Schema/Diff/Diff.php#L165. As result indexes cannot be removed unless declared in db_schema*whitelist.json. In fact, index removal is not destructive as it does not delete any data or impact any behavior. Only performance may be affected.

STR

1. Declare index on any table (existing index may be used)
2. Install Magento
3. Run **show create table <table_name>;** to verify that index is present
4. Remove index declaration from db_schema.xml
5. Run bin/magento setup:upgrade
6. Run **show create table <table_name>;** to verify that index is still present
7. Add removed index to db*schema*whitelist.json
8. Run bin/magento setup:upgrade
9. Run **show create table <table_name>;** to verify that index is removed

AR

1. Index created by declaration is not removed from DB unless declared in a whitelist

ER

1. Index created by declaration is removed when a declaration removed