I think you will agree that I am one of the 'left', right? I fully support vote by mail and requiring ID including proof of citizenship.
Oregon requires ID to register to vote and requires you register at least 21 days prior to an election in order to vote in that election. We've had plenty of time to refine mail in voting. We've been voting by mail since 1981. In 2004 there was an 84% turnout in the Presidential election. Currently, we are 100 percent vote by mail.
Vote by mail is hardly something new. As of July 2020, five states – Colorado, Hawaii, Oregon, Utah and Washington – hold elections almost entirely by mail.
In the 2016 general election 27 states had 'no excuse needed' vote by mail. Here is are some maps that show how widespread vote by mail was in 2016. I don't recall Republicans and Trump challenging the validity of vote by mail then.

Ok, but that isn't what I asked.
Obviously you have common sense on this subject, but others do not. This is why we've seen the MLB, Hollywood boycott, Biden (the left) calling it Jim Crow on steroids, democratic senators feigning outrage, etc.....So obviously there's a issue.
So, to restate, what do they feel is racist or suppressing about this law?
A read through of the law suggests there isn't any suppression, only that the left keeps repeating it that there is. I legit want to know what their argument for it is.