Let's go to the videotape!
Merriam-Webster's: slang, often offensive
American Heritage: Often Offensive Slang
Collins: offensive slang
Wiktionary: slang, derogatory, offensive (The offensiveness of this term and of spastic differs throughout the Anglosphere. In the UK, Ireland, Canada and Australia, it is highly offensive. The term is more casually used in the U.S., but is still offensive to some disabled people)
Cambridge American: informal offensive (an extremely offensive word for a stupid person, or a person who behaves or moves in a strange or uncontrolled way. an extremely offensive word for a person who has cerebral palsy)
OED: slang. offensive (usually derogatory or depreciative) (In British use typically derogatory and always closely associated with the meaning in sense 3, and as a result considered highly offensive to people with disabilities. In North American use typically mildly depreciative and not considered highly offensive before the 21st century.
Dictionary.com: (The slang use of spastic (or spaz ) to mean “clumsy” is perceived as insulting to people who are affected with muscular spasms.)