Same here..... I've been using Free Account for more than a decade on multiple devices.
To bypass restriction using multiple devices, I had configured all my PC/Laptop instances of Boxcryptor to use a LOCAL key, This is working just fine in combination with Google Drive. So I do not see WHY it should stop working once they shutdown their servers.
Nevertheless, I won't take risk, and I'm now looking into Cryptomator (+Gdrive). It is open source + it has an Android client. So a perfect replacement of Boxcryptor.....