I have the same issue, but I looked in previous questions about QC and in 2015 someone answered with a workaround:
select the pic you want to go into the QC and press "b" which means "bump" apparently, and it adds it to the QC. You can add multiple selected images at one time.
In fact, now that I was searching for something else, I see that in the LR menu at the top under "photo," the first entry is "add photo to Quick Collection B"
Target collection options seem to be grayed out.
BTW, I don't see the Filter icon at the top or the flag. I have Adobe CC and I believe it is up to date. Perhaps.
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Quick collection is still here. It is in the update to Lightroom. Adobe now calls Lightroom Lightroom Classic. Lightroom CC is a completely new product which is neat because it stores all images in the cloud, however it misses about 90 % of the features present in Lightroom proper. It is basically a port of mobile Lightroom to the desktop.
"however it misses about 90 % of the features present in Lightroom proper. It is basically a port of mobile Lightroom to the desktop."
That actually explains a lot about the Cloud offering. I thought I was missing something obvious..