It's been common among musicians long before rock'n'roll was developed. A lot of musicians killed themselves the old-fashioned way, with alcohol.
Never got into STP; it just didn't play right to my ear. But they have talent. Had. I may not play the songs over and over again, but I know a good voice when I hear it. And, you know, I also admit this is not unexpected. Sadness ensues.
You mean "junkies." A junkie is an addict. It is derived from "junk," an old slang word for heroin. Junky is strictly an adjective, meaning "in very bad condition," "poorly built," "not sturdy enough," etc. It is derived from the original meaning of "junk," something that is no longer useable and has been discarded. Its use as slang for heroin (or other narcotics) arose about 90 years ago, but it's not clear why that particular word was appropriated--this is common with older slang words. Today everything is on the internet so it's not hard to determine the origin of anything. Dope is an older slang word for narcotics. It refers to the way heroin and some other drugs were prepared as a paste. "Dope" is a synonym for paste.