I think supporting this mechanism in dh-update-R should be fairly easy, so it's really a policy question of what an appropriate compat level and compat-mechanism is preferred for R packaging.
Debhelper 12 is available in buster, stretch-backports (and bionic-backports or later ubuntu). My inclination is that it is cleaner to not have the d/compat file and just have it inline in d/control. I'll see if I can come up with a patch for dh-update-R which supports this mechanism. After that we can decide if it should be made the default for new and/or updated packages.
