They are still providing updates for a 5 years old model, which is longer than any of their competitors do at the moment. For instance, check Netgear's support for the R7500 V1, which is their Quantenna-based product similar to the RT-AC87U. They dropped it, and only support the V2, non-Quantenna based revision.
EDIT: I just checked, Netgear last update the R7500 V1 in Mid-2018. That means they dropped support for it 20 minths ago, while Asus still provides support for their RT-AC87U.
I simply don't have the time and resources to provide separate legacy support for these models, since this is a one-man gig. In an ideal world, someone would have a fork that is still based on the 382 code, and be able to maintain that in parallel to the 384 models that I currently support. That 382 codebase would allow them to keep merging in updates from Asus.
Unless Asus decides to move these models to the 386 code base at the same time they move all others, I will have to entirely drop support for these in the near future as I simply don't have the time to deal with the extra workload involved in maintaining three separate branches.