I quickly got tired of trying to find an answer in the docs, so I asked ChatGPT:

Content observers, which allow apps to listen for changes in data in a content provider, are also affected by the changes in Android’s background processing restrictions.

Starting with Android 9 (API level 28), the system limits the frequency at which content observers receive notifications about changes in data. Specifically, content observers are now subject to the same background processing restrictions as other types of services and apps.