There is plan to support jdk15u for some longer time, much like jdk13u (medium time release). It would be more convenient with Skara, Git, and GitHub.
So, could you please, if possible, convert jdk15u to Git?
I think it would be right to keep the setup similar to 13u (and basically 11u and 8u) with a team repository jdk15u-dev and a master jdk15u.
The team repository jdk15u-dev may be converted around March 22, 2021 and opened for (July) release 15.0.4. The master could stay in Mercurial, in 15.0.3, for some time after release scheduled on April 20, 2021, and converted as soon as the April release is finished.