The latest news on this front comes from a really interesting article at Ballot Access News written by the widely-respected election expert and advocate for third party/independent ballot access Richard Winger. Take a look at this excerpt:

Run for Something, an organization formed in 2017 to encourage young citizens to run for public office, has released a study that shows the value of a party running for seats that it knows in advance it will not win. The study concerns Democrats in the south in 2020. When the party ran a nominee for state legislature, even though the seat was considered completely safe for the Republican legislative nominee, that boosted the party’s share of the vote inside that district for president and other statewide office. See this New York Times story.

Run for Something has a website, but unfortunately does not seem to display the study on its website. If any commenters knows how to find a copy of the study, please comment. UPDATE: here is the study. Thanks to Samuel Wang for the link.