Softwares like Internet Explorer 3 and Photoshop 1.0.1 are on-topic. They are considered retro and their codebases only partially resemble the modern versions. However, a consensus about software that has not been removed from the codebase has not been reached.
Case study: Windows Explorer Shell Command Files
A feature added to Windows with Internet Explorer 4 (95 / NT 4).
.scf files behave like shortcuts in the shell - they can't be opened in programs using the built-in Windows open dialog's GUI - only by typing in the full file path. These files were used in the now-defunct Quick Launch bar to show the desktop, open IE4's Channels feature and open Windows Explorer(?). These files have not been created by Windows since XP.
Case study: Macintosh QuickDraw API
Any Mac OS GUI developer will remember QuickDraw. This revolutionary graphics API allowed people to not only draw lines, rectangles, circles, images, text and more, but also allowed them to use Apple's patented "regions" technology. Apple implemented it into Mac OS X for backwards-compatibility; until Mac OS X v10.8 one could still compile programs using QuickDraw, and in v10.8 it was only removed from the SDK; QuickDraw programs would still run.
Are such software features on-topic?