Texinfo ultimately gets the @ convention from Brian Reid's Scribe[1], as developed at Carnegie Mellon during the late 70s and commercialized by Unilogic[2,3] in the 80s. Coincidentally, there was a close derivative of Scribe called Mint[4], also developed at Carnegie Mellon in the early 80s for the PERQ (an early personal workstation competing in the category of things like the Sun-1 or Lisp Machines).
Was this an independent hobbyist reimplementation? Gobe did something similar in order to port their productivity suite (sorely missed) from BeOS over to Windows and Linux.
Had juist a hobby. Never released it, but it worked quite well. It was remarkably easy to add the events and controls etc after I got the initial version working.
I believe the implication of OP's comment is that Doom and Wolfenstein 3D were originally released under non-commercial source available licenses before being relicensed to a FOSS license (GPLv2) a few years later, and the same could very well end up happening for Rogue Legacy as well.
The classic book references for programming shmup bullet patterns (in Japanese) are シューティングゲームアルゴリズムマニアックス and シューティングゲーム プログラミング, but in general, a lot of this stuff is folk knowledge and isn't communicated well outside of trial and error until you get something passable. I would suggest looking up web tutorials and game source code and learning from that (also will mention the Development sub forum on shmups.system11.org being quite useful).
If I had to wager a guess, probably something like labwc would end up in that role for a distro of DSL's scope and philosophy. However, I think we're still a while away from that, and for the targeted machines, X is still completely fine and will remain so for a long while.
I wonder if there will be any substantial increase in pedojacketing among cultural elites towards those they see as undesirables now that the bill is passed, or whether the second order effects of the bill will lead to a backlash against the existing proliferation of it from the young, globally connected, and tech savvy populations still remaining in Britain after the depredations of the last decade.
For ARM32, RISC OS is probably the closest to the value proposition offered by Menuet, albeit with a decent amount of high level code and a lot more support from developers for applications, languages, and games.
Xanadu comes close to this "ferengi Internet" mindset with some of the tactics it chooses for monetization of content, albeit from an entirely different angle (enabling remix culture more or less indiscriminately while preserving the sanctity of the existing copyright system and enabling royalties to flow to authors proportional to how their works are used and reused).
It's not mentioned in the article, but the author uses AppImages for GUI applications wherever and whenever he can. He's very well aware of this aspect of the classic Mac OS and seeks to emulate it as closely as possible.
[1] https://bitsavers.org/pdf/cmu/scribe/Scribe_Introductory_Use... [2] https://bitsavers.org/pdf/unilogic/Scribe_Document_Productio... [3] https://bitsavers.org/pdf/unilogic/Scribe_Pocket_Reference.p... [4] http://www.bitsavers.org/pdf/cmu/spice/Users_Manual_for_Mint...