[Edited] Canonical has not released any official statement, or at least I have not seen it at the time of writing, but what appears to be the first beta of Ubuntu 19.04 Disco Dingo is now available and all its official flavors. The first (I saw) to break the news was Ubuntu Budgie via Twitter, who was joined by Kubuntu hours later. And the page which shows the downloads is new, or is it for I was disconnected from these types of releases for a while.
In the mentioned download page there is images of all official Ubuntu flavors, but not just desktop systems. If they were only this type of images, there would be 8, those of Ubuntu and its 7 flavors, but we can also see that there are images of Ubuntu Server, Netbook and Ubuntu Base. In total, there are currently 28 Disco Dingo ISO images, all numbered 20190326.1, which means it is the first version as of March 26, 2019.

[UPDATED] It seems like, This is not the first beta, but what could become the first beta of Ubuntu 19.04 that is scheduled for tomorrow Thursday. The confusion comes from the tweet you have below, where Ubuntu Budgie says that «The first beta ISOs of 19.04 Disco Dingo are now available«. On the other hand, it is Kubuntu who says that «Kubuntu is gearing up for our Disco Dingo beta release on Thursday. Candidate ISOs that could become beta are now available«. In fact, the confusion is more than logical taking everything into account, among which we have the previous image.
Ubuntu 19.04 will have a support of 9 months
First 19.04 Beta Disc ISO's are now available. Fancy helping out testing? Simple "follow the instructions" test cases. Just login with your launchpad ID. Go on - you know you can do it :) https://t.co/JWvhpxIgNJ
- Ubuntu Budgie (@UbuntuBudgie) March 26, 2019
Ubuntu 19.04 it won't be the flashiest release in recent years. Basically, when we have to write about it, the same thing always comes to mind, which will come with Linux Kernel 5.x, but little else. It was expected to come with Android integration thanks to KDE Connect or GSconnect, the GNOME version, but it seems that this option has finally been discarded. Although it remains to be seen what will arrive, everything seems to indicate that most of the changes will be internal, as well as that they will arrive with new versions of GNOME, KDE, LXDE, Xfce, MATE, Budgie and the rest of the graphical environments.
Ubuntu 19.04 Disco Dingo will officially arrive on April 18. Personally I would recommend updating to all those users who are on version 18.10, since it will come with the latest kernel news, themes, etc, but I would not recommend it far from it for those who are in an LTS version, we remember that the only ones with official support are Ubuntu 16.04 and Ubuntu 18.04.
Are you going to test this beta? If you do, feel free to comment on your experiences.