This site is the jumping off point for tons of areas in research and information about Missouri history.
If your research includes someone who died in Missouri from 1910-1969, you may be able to see their death certificate here. It includes much family information and history.
https://s1.sos.mo.gov/records/Archives/ArchivesMvc/DeathCertificates/
More than 9 million records can be accessed through Missouri Digital Heritage, including the collections of the Missouri State Archives, the Missouri State Library and other institutions from across the state.