there are states that only do vote by mail. oregon being the first to go there in 1998.
https://www.nbcnews.com/politics/elections/you-can-t-hack-paper-how-oregon-fights-election-meddling-n930481
Washington and Colorado also have mail-only elections, according to the National Conference of State Legislatures. Twenty-two states promote some elections as "all-mail," but they don't preclude in-person voting, the conference says.