Can you still make custom hero builds in dota 2? Ever since the reborn update, I havent found a way to do so... is it simply in another place that I havent looked yet? Any Ideas?
Yes, you can still create and edit builds on the official Dota 2 website.
Yes, you can still (partly) create builds in game. When in a match (or private lobby) you can click the edit icon in the top left of the default hero build section, which then you can drag items in and out of the build section.
But As @Steffo said, now you can only get the full hero build features like ability upgrade path and item/ability notes through the website. And there is a problem with modern browsers where you can't hover over text boxes to input the notes, you have to tab to them and type, then tab-enter to the save button otherwise the text box disappears. I hope they fix this soon and add back the build editor in game.