IIRC, they changed it to get more value out of the 260 character MAX_PATH. I know there was some sort of manifest to enable longer paths, but I'm not sure what the current status is.
The status quo is that officially, you still have to both set a registry key (or equivalently, set an option in the Group Policy Editor) and add an element to each application manifest.
The official workaround at runtime is to use the "\\?\" prefix with an absolute path to create an unrestricted verbatim pathname. For instance, the fs::canonicalize() function in Rust will always return such a pathname, to many programmers' dismay, since outside tools often choke on them.
The unofficial workaround is to set the undocumented IsLongPathAwareProcess bit in the process's PEB. The Go runtime does this, but silently falls back to "\\?\" prefixes if the Windows version is too old.
(Note that in general, canonicalizing paths is safer on Windows than on Unix-like systems, since open directories cannot be renamed.)
