OpenSim 0.9.3.0 supports PBR. OSgrid's official sims, the Plazas, are all PBR-capable as has been proven. I've seen a picture made by Hicks Adder showing a working mirror at one of the Sandbox Plazas, and I suspect that beat-up sci-fi helmet on display at Lbsa Plaza to use PBR.
As far as textures go, both SL and OpenSim have a fallback: Every surface can have both a set of PBR textures and a set of traditional non-PBR/Blinn-Phong textures. The latter are used in non-PBR viewers, the former are used in PBR viewers. You only get grey surfaces if the traditional textures are missing.
In addition, OpenSim also has fallback textures for the ground. Second Life doesn't; it doesn't seem to need them. After all, two versions from now, the non-PBR versions of both the official viewer and Firestorm will no longer be allowed. OpenSim doesn't have this rule, and there are still people using Firestorm as old as 6.0.2 although it doesn't support BoM, and BoM has no viable fallback.
As for the demand for PBR, it will come. Just like BoM came. For now, there won't be much happening, what with how few places support it, how rough around the edges the first PBR Firestorm is, and how few people actually know what BoM is. It's similar to the spring of 2020. All places that ran on vanilla OpenSim had at least basic BoM support, but Firestorm 6.3.9 was so buggy that everyone stuck with Firestorm 6.0.2 with no BoM support. The only BoM content available were a few basically unknown Ruth 2.0 or RuthToo forks with basic BoM and, released in May, Roth2 v2 with scripted BoM which nobody would ever had heard of either if I hadn't peddled it.
Then, in July, came Athena 6 which most OpenSim users take for the first BoM body in OpenSim, in August came Adonis 4, both with very basic and actually incomplete BoM, and in autumn came Firestorm 6.4.13 which fixed Firestorm 6.3.9's show-stopper bug. Still, nobody really knew what BoM was, and even only few people stopped using skin appliers and actually started using BoM. But that teal "Free Bakes on Mesh" label became synonymous for "the newest hottest shit you can possibly put on your avatar". Still today, hardly anyone really fleshes BoM out, also because nobody dares to rip new layer clothes from SL because they'll be perceived as ugly, out-dated crap.
For the near future, I can see two things happen. One, at least one new Firestorm version irons out the rough edges of 7.1.9. (Sorry, no way back to having forward rendering without ALM.) Two, someone starts ripping shit-tons of PBR content from SL and offering it in their freebie store with a brand-new, custom PBR logo on it. Either that'll happen at iPleasure or Darkhearts, boosting either even more, or that'll happen on an entirely new sim which has the chance of becoming more popular than iPleasure and Darkhearts.
Either way, while still nobody will know what it actually means, PBR will replace EvoX as the label for the new hotness, just like EvoX replaced BoM. Also, nobody will really flesh PBR out because, just like now, nobody will be bothered to install light sources anywhere on their sims.
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