At the moment i’m moving my own, local, production environment from 0.5 to 0.6 and start supporting v0.7 for the packages (two) i maintain: Cairo and Rsvg. And i somehow have the feeling nobody told me, that we have decided to drop v0.4 support. First it was only tkelman’s comment on one commit and later (today) i learned that Compat is in the process to drop 0.4 completely - well actually they already did.
So is this the situation?
I think that’s right. Download Julia says:
For Julia 0.5, only bugfixes are being supported. Releases older than 0.5 are now unmaintained.
This i knew. But support of the julia development and support by packages are a different story.
That’s really up to package maintainers, but I think the general trend is indeed to drop support for Julia 0.4. Having to support Julia 0.5, 0.6 and (sometimes) 0.7 in parallel is enough work. 
I can understand the delegation to package maintainers, but the decision in Compat enforces this in quite many places.
At some point the 0.4 METADATA is also frozen: one can no longer update packages with minimum julia version of 0.3, for example. But I don’t think this has happened yet.