Mr Red, not sure I understood. Psychedelic is in my 1996 Webster's Collegiate. Obstreperous also is there as it should be; it is an old word as least as old as 1600, and fairly common in usage. Never heard of obstro---.
Mr Happy. Gullible- dating at least back to 1800, is in Webster's. Commonly used words are NOT removed. It is a word heard every day.

Some words are removed from smaller dictionaries. I have an old Webster's Collegiate, from the 1940s. In it are a number of Scottish words that are absent in the newer Webster's. They have been removed to keep the size down. They would not be removed from a comprehensive dictionary (the OED). The OED just keeps getting larger.