Using Linux portable version 67 of ungoogled-chromium on an up-to-date Calculate Linux (gentoo) installation, once chrome extensions are downloaded into a folder to be installed as per the method suggested in the FAQs, they could not then be dragged successfully into the Extensions tab in ungoogled-chromium.

That version was downloaded from the #414 (comment) to /tmp, untarred and moved to /opt/bin without changing permissions or ownership:
tar -xvf ungoogled-chromium_67.0.3396.62-1_linux.tar.xz

sudo cp -r ungoogled-chromium_67.0.3396.62-1_linux /opt/bin/

The directory was renamed (optional):

sudo mv /opt/bin/ungoogled-chromium_67.0.3396.62-1_linux/ /opt/bin/ungoogled-chromium/

Attempting to drag and drop into the Extensions tab produces error message in tab: 'Apps, extensions, and user scripts cannot be added from this website'.

Is this an unfortunate development with Chrome/Chromium upstream? Are developers not appreciating ungoogled-chromium and like derivatives?

A workaround to install extensions was successful for Dark Night Mode extension but failed in part on first attempt and did not load during the following session with the only other extension tried - HTTPS Everywhere:

1. Rename the extension's extension (in other words, its suffix) from .crx to say .zip
2. Unzip the file into another folder, say, unpackedextensionNo1 to keep the zip file and its contents separate
3. Open Extensions tab in ungoogled-chromium
4. In ungoogled-chromium's Extensions tab, turn on Developer mode
5. Click on LOAD UNPACKED
6. Navigate to folder and select it (Click on Open) to install.