Unfortunately, I'm unable to open a support ticket now because our contract has been suspended due to some political reasons. If I had a chance, I'd definetely done it first.
The same obstacle with upgrading to 2406 cause it requires to upgrade the license on the Omnissa portal.
Futhemore, I've tried to look through all of the release notes for all the versions of Horizon and couldn't find anything about REST API in resolved issues.