Yes, you can still drop off your ballot at the library, and lots of other drop off sites.
Visit the Multnomah County Elections website for the list of all August 11, 2020, Special Election ballot drop off sites, including a list of which Multnomah County Library branches (and the Central Library, downtown) are open for business and accepting ballots.
By the way, if you want to look at some May 2020 Primary Election voter statistics, visit the Oregon Secretary of State Elections website for lots of reports, including this one:
VOTER REGISTRATION AND PARTICIPATION AGE DEMOGRAPHICS BY COUNTY
MAY 19, 2020, PRIMARY ELECTION. (Age demographic data by county begins on page 26, but the entire report is interesting.)