So, I’ve been so busy recently that I’ve not had a chance to actually “use” Magento 2.3 since I did the video last year. My sandbox server still has a clean install that’s not been touched.
I’ve just gone to upload a Logo and have the exact error that you’ve shown. At first, I assumed that the issue was just because the appropriate folders didn’t exist. It turns out, this isn’t the case.
It does in fact look like a Bug in Magento 2.3 (Ref this article). I’ve tested the “workaround” and it does fix the issue. Although, I wouldn’t recommend the method as it means editing core code. If you’re testing/playing then there’s technically no harm, especially as this file will be overwritten when you next patch Magento anyway.
I’m just shocked that they’ve left this bug sit here for so long. I think that’s nearly 2 months since release. Another reason why I don’t upgrade my Production environments to major versions until the patches come out.