Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\Software\Policies\Google\Chrome]
"ExtensionSettings"="{\"*\":{\"runtime_blocked_hosts\":[\"*://ycombinator.com\",\"*://boh.com\"]}}"
Tested in a new Windows 10 x64 (2004) VM with a new install of Google Chrome 81.
Thank you - it must be me. Had the same issue in a macOS 10.12 VM with Chrome 81. Here's a screen recording of the issue in the Windows 10 x64 (2004) VM: https://tinyapps.neocities.org/cg.mp4 . Tried reloading policies in chrome://policy and waiting 15 seconds as well, but there was no change.
I've played with it fairly extensively in a brand new Windows 10 VM, trying various permutations, but still cannot get it to work as intended. superasn's suggestion[0] to set site access permissions via Preferences > Extensions > Details (for the extension in question) > "Site access" worked a treat for me.
Here is the generated reg file:
Tested in a new Windows 10 x64 (2004) VM with a new install of Google Chrome 81.