This proposed tagline and pictorial representation miss the mark in conveying the (current/new) site scope and the new name, "Software Engineering."
While I agree that the site scope should include software development lifecycles and more generally software development processes, methods, and methodology, I propose that we not use the terms "System Development Life Cycle" or "SDLC" nor link to Wikipedia's SDLC page to help explain SE.SE. That part of the Question is what goes off the mark for me.
"Systems development" implies projects where software is one component along with hardware, courseware, wetware, etc. E.g. space systems and robots (ME+EE+SE). (The phrase "software systems development" doesn't add anything over "software development", and shortening it to "systems development" makes it ambiguous between systems of vs. including software components vs. "systems software".) I propose that systems development topics not become in scope for SE.SE.
"SDLC" may be construed as a broad term for "any kind of development lifecycle" or as a subcategory of lifecycles which contrast with incremental approaches. Wikipedia's SDLC page is inconsistent about that, as are other sources. First it says Waterfall and Agile are among the SDLC models, then it contrasts SDLC where documentation is "Vital" (each phase needs a work product and may end with a team handoff) vs. Agile. The page focuses on stages of work, like building components then integrating them, never mentioning continuous integration.
- broaden the scope to Systems development [as in the SDLC references]
- narrow the scope to discussions about software development lifecycles themselves [as in the tag line phrase "questions directly related to the SDLC" and the proposal "Please make sure that your question is directly related to the Systems Development Life Cycle."]
- suggest that the Waterfall lifecycle is desirable
- imply that the scope includes the details of testing, operations, and everything that's within the development lifecycle except coding and debugging [as in the SDLC Waterfall diagram with those parts crossed out]
The referenced SDLC picture is a Waterfall diagram except the sequential steps are drawn ascending rather than descending. Waterfall is the simplest complete model but it maximizes risk of failure by delaying empirical feedback. It's responsible for many big project failures.
From The road to Agile:
In 1970, the first article on the Waterfall model was published by Winston W. Royce. Royce did not use the term Waterfall in that article, but he presented the model as an example of a flawed, nonworking model.
What is software engineering about? Let's look at the ACM professional organization [my italics]:
Software Engineering Notes (SEN) is an informal publication of the ACM Special Interest Group on Software Engineering (SIGSOFT) concerned with the cost-effective, timely development and maintenance of high-quality software. Relevant topics include requirements, specification, design and implementation methods, software maintenance, reuse and re-engineering, quality assurance, measurement and evaluation, software processes, automated tools, practical experience, and related issues.
Engineering is about achieving quality attributes like performance, security, modifiability, reliability, usability, maintainability, safety, timeliness, cost-effectiveness, portability, and testability.
Engineering is about applying principles when building things. Many principles are known, in categories such as mathematics, empirical evidence, scientific knowledge, and practical know-how.
The site's tagline (draft 2) would then go something like:
Software Engineering Stack Exchange is a Q&A site for professionals, academics, and students in software development and related fields who are interested in getting expert answers about applying principles to building software systems and achieving quality attributes such as performance, safety, reliability, usability, maintainability, and testability. It is not a site for code troubleshooting.
Or a shorter version using the word "methodology" both in the sense of "a system of methods" and "the study of methods":
Software Engineering Stack Exchange is a Q&A site for professionals, academics, and students in software development who are interested in getting expert answers about software development processes, methodology, and achieving qualities such as performance, safety, reliability, usability, maintainability, and testability. It is not a site for code troubleshooting.
Draft 3, aiming to address @ricksmt's central issue about getting the point across to people currently asking off-topic questions:
Software Development In-the-large Stack Exchange is a Q&A site about software development "in the large" including development processes and methodologies for teams, reliably meeting requirements, and achieving quality goals such as safety, reliability, usability, and testability. It is not a site for code troubleshooting.