They've been doing this for a while. This post is headlined "One Last Time".

In this 2018 link, 23rd in a series, the LoC says it has "exhausted its currently slate of unidentified stills"

So, they have very few unidentified stills in the Moving Image and Recorded Sound division of the Library of Congress.

https://blogs.loc.gov/now-see-hear/2018/01/photo-blog-23-fin...