It could be as simple as one of the maintainers still using the S5 as their primary device (or is being sponsored to support it) but none of them are using an S

It could be as simple as one of the maintainers still using the S5 as their primary device (or is being sponsored to support it) but none of them are using an S6. Or there could be a driver issue with the S6. Or something else entirely. The major brands don't really care all that much about unofficial builds like LineageOS and really wish they'd just go away which often makes life more difficult for maintainers. (they should be careful of what they wish for: the momentum to support many of these devices seems to be waning as people move the the up and coming manufacturers) If the S6 is still on 14.1, it's not likely it's going to get 16.0 support... I'm in the same boat with my older Nexus devices.