MutableCapabilities
MutableCapabilities was introduced for the first time with the availability of Selenium (Java Client) v3.6.0
Here is the snapshot from the Selenium (Java Client) v3.6.0 Release Notes:
All Option classes now extend MutableCapbilities
new RemoteWebDriver(new ChromeOptions());

With the availability of Selenium (Java Client) v3.7.0 we migrated from using DesiredCapabilities to either
MutableCapabilities or (preferably) ImmutableCapabilities.
Here is the snapshot from the Selenium (Java Client) v3.7.0 Release Notes:
Migrated from using DesiredCapabilities to either
MutableCapabilities or (preferably) ImmutableCapabilities.

org.openqa.selenium.MutableCapabilities
Here is the JavaDoc link of org.openqa.selenium.MutableCapabilities