Visiting the front page shows
MediaWiki requires the PSR-3 logging library to be present. This library is not embedded directly in MediaWiki's git repository and must be installed separately by the end user. Please see mediawiki.org for help on installing the required components.
which has been talked about here before.
I updated composer.lock to remove a line and then ran "composer update". Yes, I know, don't do that, but I did. vendor has psr/log/PSR/Log and composer.lock that refers to psr/log.
The vendor/composer/autoload* files exist (I erased them so they would be regenerated( but none refer to any of the installed libraries.