From the perspective of the user at least, the dm reference is living document. Deprecations and additions are indicated with in-line notes and version tags. In some cases this is pretty squirrely.

It would be nice if
- major versions had documentation that does not include future additions or past deprecations.
- migration notes were included in each major version's documentation.
(for example, hunting down https://www.byond.com/forum/post/2828780 is not necessarily obvious)

I envision a pattern like

byond.com/docs/ref/514 - documents 514.
byond.com/docs/ref/514/upgrading - breaking changes from 513 to 514.

byond.com/docs/ref/515 - documents 515 until it is frozen.
byond.com/docs/ref/514/upgrading - breaking changes from 514 to 515.

byond.com/docs/ref - Includes the current stable in an iframe or something. Has links for navigating to other major versions.