Changes in Second Life inevitably have to be taken over by Firestorm for Second Life. If Firestorm for Second Life changes something, then so does Firestorm for OpenSim. There's only one common codebase for both; they're just compiled using different build flags. And if Firestorm for OpenSim changes something, then OpenSim itself has to follow suit in order to still have a good, compatible viewer.
Thing is, when Second Life introduces a new feature, it currently tends to also remove an old feature. This means: Firestorm for Second Life has to remove it because it can't co-exist with the new feature there. If Firestorm for Second Life removes it, it also disappears from Firestorm for OpenSim. And this means that OpenSim itself will have to change accordingly.
This happened when EEP was introduced: Windlight was removed because it couldn't co-exist with EEP.
PBR was not quite the same, but similar. PBR was also taken as a chance to reduce the complexity of meshes in Second Life. I mean, Second Life had already supported Blinn-Phong normal maps that simulate bumpy surfaces or small surface details. It has optionally been part of the Advanced Lighting Model for many years already.
But hardly any creator used them. Instead, they kept building their stuff for potato computers, for the old Forward Lighting which doesn't support bump maps or normal maps. They added surface bumps and surface details to the meshes, making them ridiculously complex, so that their creations look great on absolute toasters with on-board graphics at the lowest graphics settings possible. These super-detailed surfaces actually bogged computers down more than a bump map or a normal map could.
So what Linden Lab did was: When PBR was introduced to the official viewer, the Advanced Lighting Model was hard-coded to on. The old and simpler Forward Lighting was fully discarded. The one justification for content creators to build with 500 surfaces what could just as well work with two surfaces plus a bump map was removed. And the Lindens told the Firestorm devs to follow suit if they still wanted Firestorm to be allowed as a Second Life viewer.
Since Firestorm for Second Life had to introduce PBR (to please SL users) and remove Forward Lighting and turn the ALM permanently on (to please the Lindens), Firestorm for OpenSim had to do the same. Again, one and the same codebase.
And since Firestorm for OpenSim had to introduce PBR, OpenSim itself had no other choice but to introduce PBR, too.
By the way, there used to be a time when Firestorm for OpenSim had its own codebase. It saw exactly zero maintenance. Firestorm doesn't have dedicated, specialised OpenSim devs. They've got 20 developers. 19 of them are only in Second Life. Only one is in both Second Life (primarily) and OpenSim (when necessary). This ended with two options. Either Firestorm for OpenSim is discontinued altogether. Or it's merged with Firestorm for Second Life again. Fortunately, they did the latter.
Another by the way: Firestorm 7 doesn't require brand-new, top-of-the-line graphics hardware. I'm using upper-mid-range hardware from 2018. Six-core Ryzen, 16GB of RAM, Radeon RX 590 with 8GB of VRAM. Anyone with an actual high-end gaming rig would call this a hopelessly outdated toaster. But it runs Firestorm 7 with ALM permanently on just fine, even with shadows on. And you don't have to fire up all PBR bells and whistles at parties.
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