URL parameters are additional query strings that follow the main URL for a page, typically contained after a question mark. Multiple query string parameters can be used within a single URL, which can impact a website’s SEO and organic search performance in several ways, including causing duplicate content and crawling issues.
This collection of SEO Office Hours notes addresses common problems seen with URL parameter handling, with recommendations on how to fix these issues with best practices suggested by Google Search Central.
URLs with hashbangs won’t be indexed, so if they are required to produce unique pages, you’ll need to migrate to a traditional URL structure without the hashbang.
John recommends against robots.txt, because it prevents Google consolidating authority signals, but then says there are occassions when crawling efficiency is more important.