Question

Why is the DOMSubtreeModified event deprecated and what are we supposed to use instead?

Answer 1

If you scroll down a bit, you see:

Warning! The MutationEvent interface was introduced in DOM Level 2
Events, but has not yet been completely and interoperably implemented
across user agents. In addition, there have been critiques that the
interface, as designed, introduces a performance and implementation
challenge. A new specification is under development with the aim of
addressing the use cases that mutation events solves, but in more
performant manner. Thus, this specification describes mutation events
for reference and completeness of legacy behavior, but deprecates the
use of both the MutationEvent interface and the MutationNameEvent
interface.

The replacement API is mutation observers, which are fully specified in the DOM Living Standard that supercedes all of the DOM level X silliness.