Support for option 121 was added recently:
2200. [func] piotrek
Kea now supports new DHCPv4 option code 121, Classless Static
Route option defined in RFC 3442.
Option may be defined in binary format and also as a string
using convenient static route notation.
(Gitlab #3074)
Regards.
Francis Dupont <fdupont at isc.org>