Do prices of older iPhones decrease after the release of a new model? I want to buy an iPhone 8. However, in a few days new iPhone 11 will be released. Can I expect older models to become cheaper at that moment or soon afterwards? Or does the price drop happen before the release?

Since shopping questions aren’t encouraged here - the scope of this is official Apple list price - historical behaviors, and not speculation about some future Apple release cycle even if one is imminent.
During the iPhone 11 announcement, Apple said they were reducing the price of the iPhone 8 to starting at $449 and the iPhone XR to starting at $599. These price changes are currently effective on Apple's website.

Historically, Apple announces new models and pricing and at the same time, refreshes their official pricing the same day as the keynote / press release. Additionally, look online and at Apple stores as they then sell (often at further discount) any now “discontinued” models in the Certified Refurbished / Clearance section: 


https://www.apple.com/shop/refurbished