Clarification here: they are asking you to not, and warning that you're getting into "unsupported territory".
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
The GPL clearly states that there needs not even be a particular purpose.
I think what they mean by supported though is (in a very simple example) if you complain about a missing button but it's because your DE theme washes it out then they aren't going to help, otherwise they'd look at the issue and try to resolve it. I think this is reasonable. Sure there's no awareness, consideration, capacity and all that but a reasonable attempt at support.
At that point they are the ones abusing the OS environment to gain benefits without contributing back to the community. And it becomes hypocritical to say "you can't do what im doing"
> We understand the need for distributions to stand out. However, we urge you to find ways to do this without taking away our agency. We are tired of having to do extra work for setups we never intended to support, just to have that used against us when people tell us the breakage from theming is “not that bad”. You are not doing this to Blender, Atom, Telegram, or other third party apps. Just because our apps use GTK that does not mean we’re ok with them being changed from under us.
> Since you are shipping the GNOME platform, we assume you want this ecosystem to be healthy. If you do, we ask that you please stop theming our apps.
I'm sorry but just becuase you make some software that works together you don't get to designate it a platform and decide you should control it. Not in the free software world, at least.
Debian et al. aren't "shipping the Gnome platform", they're shipping package managers that can install many things including a bunch of Gnome software (and vastly more non-Gnome software). A very large proporrion of it can be themed or customised in some way.
Some people use Gnome "stock" but don't install or use any of the Gnome apps, others use aspects of the Gnome WM but without the default panels and interfaces, others install the whole lot, some run Gnome software but with a totally different WM or desktop environment, and then there are things like Regolith [1] which use some Gnome software but are pretty unrecognisable compared to a default Gnome install.
Gnome devs are just writing software. Their desire to deifne and own and control a platform is their problem, not ours.
Honestly, this is probably the best approach. It puts the onus back on the person making the modifications for things that they know and understand that are fundamentally trivial in nature.
> I'm sorry but just becuase you make some software that works together you don't get to designate it a platform and decide you should control it.
I don't see anything about this post as an attempt to exert control. I think it's pretty reasonable to ask that users don't theme their apps with the implication that if they do they're not going to get any support. But to your point they should probably just come right out and say that.