Yes, OmniSharp decided they no longer wanted to support “older” implementations of C#, and apps like Unity are usually 1 to 3 versions behind the current C#/.NET standards. One of the comments in the thread I posted details how to tell OmniSharp to work with the older code base, but as I said, it seems like a potluck on whether or not it works.
Three years of helping students with Unity, and every time I figure out the changes Visual Studio Code or OmniSharp has made to make it not work with Unity, they change it again. We’re having an internal debate about continuing to recommend it officially (we use it in the videos because Visual Studio Code works on PC, Mac, and Linux. With Linux users, not many options, with Mac, Visual Studio Community now works on Macs).