The answer is that there appears to be a problem with Catalina creating an installation app for other versions of OSX-certainly with El Capitan from the issue here. Solution was to follow the Apple supplied procedure https://support.apple.com/en-sg/HT201372 but on another version of OSX-Sierra worked in this case.
The bug, which apparently affected 10.10 (Yosemite) and 10.11 (El Capitan) only, has been fixed in Google Chrome 79.0.3945.88.
The changelog doesn't mention it though.
So download the latest installer, install it, remove any method to block Google updates¹ and be happy again with a working Google Chrome browser.
¹: I disabled access of ksfetch/Google ...