Looking at the 2 YANG modules, they are in a good shape and I found only a few nits (without having knowledge of the ISIS routing protocol). However, the XML and JSON example YANG data are invalid, which needs to be addressed.

ietf-isis-link-attr
- leaf udabm-length - 2nd line of description has extra space
- leaf transition - 2nd line of description has extra space
- leaf unidirectional-link-delay - units mentioned in description instead of "units" statement
- uses application-specific-link-attributes-sub-tlv - second usage has extra indent

ietf-isis-flex-algo
- leaf flex-algo - range in description instead of using the "range" statement
- leaf algo-number - redundant range mentioned in the description

Appendix A invalid (yanglint used):
- none of the XML prefixes of the used identities are defined
- there are leafrefs to the ietf-te module data, which should ideally be included so that the examples can successfully be validated or at least the used ietf-te data referenced, if found in another RFC
- "At least one area-address must be configured." (/ietf-routing:routing/control-plane-protocols/control-plane-protocol[type='ietf-isis:isis'][name='default']/ietf-isis:isis)

Appendix B invalid:
- some of the prefixes used are not valid module names (iana-metric-type and iana-algo-types)
- similar leafref problems
- similar explicit validation error message