Old fashioned menu button I'm fairly new to android development, but am I long time android user. Phones used to have a "hardware" menu button, which I have since seen is emulated in software, but only when I'm running an 'old' app on my phone ("three dots stacked on top of each other that appears to the right of all the other buttons)

Why is writing apps that make use of the button discouraged?

Is it not supported by most phones?

Is it a bad user experience? ('swipe down from top to access soft buttons', etc..)
Back in 2012 google basically told developers that the Menu button was going away. Although it's not an official Android "rule" (not that I could find, at least), it's a strong Android guideline that phones no longer have menu buttons. It's a very recent development, and we should be seeing the majority of menu-button phones leaving the wild this year (my personal Galaxy Note 3 has a physical menu button but I'm due to upgrade in November). 

As for why, I honestly don't know. It's the subject of some debate, but nobody seems to be able to site an official Google source as to why they did it. Maybe it's so the bottom OS navigation bar on touch-screen only phones (like the Nexus 5 and 6, and the Moto X) could disappear without hiding app-relevant actions. Maybe Google wanted fewer buttons on phones with physical/soft button. Maybe Google just felt like it. I couldn't find any source that gave a specific reason as to why it was done.