Not all 2.8 questions are offtopic. For example this one:
Where is the Layers panel in Blender 2.8?
is legit, because it is known the layer system will change and the question will stay relevant after the official 2.8 release. And in meantime it will clarify what's going on with layers in developer versions.
There are many questions with "Something does not work in 2.8" format. They are getting closed, because it's normal behaviour until official release comes out and also they get obsolete as that "Something" gets re-implemented back into 2.8.
I should've known not to ask about blender 2.8.
You can ask about 2.8, the reason is not the 2.8 version, but the nature of the question being too narrow. You are right this is not mentioned in the Tour section, there is only:
Avoid questions that are primarily opinion-based, or that are likely to generate discussion rather than answers.
And that you get the real disclaimers what is off-topic through digging in the help section, which is much harder to get to and not many newcomers will read right away.
There is a balance between simplicity and information abundance. I don't personally mind how little the Tour page covers as there seems enough moderating capacity of this site to filter occasional off-topics in minutes. Closing mechanic is quite normal and nothing to feel bad about. I can imagine there being a link to the off-topic list from the Tour/Front page though.
You might want to see some hilarious fictional examples of what gets moderated here (some are less fictional than you would think:)
Funny Questions #1
Funny Questions #2
Funny Questions #3
2.8 questions are not that bad in general, some might be even good and legit. As they are not the biggest offender, imho they don't require special treatment on front page.