Well the front end has to be javascript. As it’s a website, so HTML/JS/CSS
Generally speaking we all use the “5” flavor but theres not “much” difference half the time.
EBS/backend services can be any language you want as you host those yourself.
In theory no, you could use the config service to store the “turn on/off” status, and the broadcaster can have a button on the config or live config buttons to send this command and the data stored in the config service and relayed over Extension PubSub
Whatever you want, your demo image looks like a video overlay. (Please note this only work on streams that are not offline, unlike you image)