Extremely strictly. They're fond of saying that most programs from v1 will Just Work in version 13. See https://reference.wolfram.com/language/tutorial/Incompatible... for a list of breaking changes. They even document certain bugfixes this way:

> AbsoluteOptions has been reimplemented to be more accurate, and the form returned for a particular option may be different now.

They deprecate stuff, but almost never remove it:

> ValueQ has been redesigned to test for the presence of a definition regardless of whether the definition is used. Old behavior can be restored by specifying "Legacy" as the method.

They play more fast-and-loose with functions documented as "experimental".