I see at Wikipedia that he had contracted COVID-19, but I don't know if that was his cause of death. He had a host of problems.
Obituaries
Johnny Crawford, who became a child star as the son of The Rifleman, dies at 75
By Harrison Smith
April 30, 2021 at 9:14 p.m. EDT
Child actor Johnny Crawford starred on The Rifleman, which ran on ABC from 1958 to 1963, with former pro baseball player Chuck Connors, who played his gunslinging father. (Everett Collection)
Johnny Crawford, who reigned as one of televisions most popular young actors while starring as Chuck Connorss sensitive son on The Rifleman, then parlayed his screen success into a string of Top 40 hits as a teenage crooner in the early 1960s, died April 29 at an assisted-living home in the Sun Valley section of Los Angeles. He was 75.
He had Alzheimers disease, said his wife, Charlotte McKenna-Crawford, and was in declining health after being hospitalized last year for covid-19 and pneumonia.
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By Harrison Smith
Harrison Smith is a reporter on The Washington Post's obituaries desk. Since joining the obituaries section in 2015, he has profiled big-game hunters, fallen dictators and Olympic champions. He sometimes covers the living as well, and previously co-founded the South Side Weekly, a community newspaper in Chicago. Twitter
https://twitter.com/harrisondsmith