There are few non-backwards-compatible changes that should not affect the majority of users. These include an updated canonical value of date-and-time value in an unknown timezone. Also, there were minor changes in a few public structures. These include changing the type of the parent member of ::lyd_node or removing the lyd_anydata_value union and value_type member of struct lyd_node_any. Both these changes made the data node structures more unified and user-friendly. Anyxml and anydata nodes now have either a data tree or string value, with string values having hints that aid in correctly printing and preserving the values.

To mention all the significant changes, the binary LYB format was improved and now fully supports 2 variants. The default one is optimized for performance and has fairly lenient restrictions on the context parsing these data. The shrank format is optimized for the size of generated data and requires the parsing context to be exactly the same as the printer context. Also, the pattern API now uses an abstraction on top of PCRE2 functions so libpcre2 no longer needs to be a dependency of every project depending on libyang.

Other changes include update internal ietf-yang-types and ietf-inet-types YANG modules to their latest revision. Then, extension plugin API has been improved and refactored to be much better integrated into the whole library. Additionally, support for OpenConfig extensions modifying pattern restirctions has been added. Finally, anydata data tree contents were integrated and are now natively supported when traversing a data tree even when using an (X)Path expressions.

Here is the overview of the changed / removed / added functions.

libyang 4.x	libyang 5.0	Notes
lyplg_type_identity_module()	::lys_find_module()	Function renamed with the same functionality.
lyd_parse_ext_data()	-	Functionality integrated into non-ext function variant.
lyd_parse_ext_op()	-
lyplg_ext_insert()	-
lyd_new_ext_any()	-
lyd_new_ext_inner()	-
lyd_new_ext_list()	-
lyd_new_ext_path()	-
lyd_new_ext_term()	-
lys_getnext_ext()	-
lyd_validate_ext()	lyd_validate_ext()	Function remained but should be used only by extension plugin callbacks. For ext data validation standard functions should be used.

The complete generated list of changes (without any additional notes) can be found in the compatibility report.