It is still quite a bit in the future until PHP 8 will be released, but as it is a major release removing and deprecating a lot of ancient features, one can not start early enough fixing things:

In Matomo:

• create_function() has been removed don't use create_function() anywhere #16181
• core/Http.php(141): Deprecated - Required parameter $aUrl follows optional parameter $method don't specify optional parameters before required parameters #16182
  https://php.watch/versions/8.0/deprecate-required-param-after-optional
• Make method signature compatible with PHP 8 #16550 Private methods cannot be final as they are never overridden by other classes in /home/lukas/public_html/matomophp8/core/Singleton.php on line 27

• remove optional parameter before required one #16312 plugins/Actions/ArchivingHelper.php on line 374
• don't log silenced errors in php8 #16313 don't log silenced errors in php8

In 3rd-party-libraries:

• PHP-DI: ReflectionParameter::getClass() is deprecated

[06-Jul-2020 13:56:34 UTC] PHP Deprecated:  Method ReflectionParameter::getClass() is deprecated in /home/lukas/public_html/matomophp8/vendor/php-di/php-di/src/DI/Invoker/FactoryParameterResolver.php on line 43

https://github.com/php/php-src/pull/5209
I still don't know DI, so I can't say much about it

Will be fixed with PHP-DI/PHP-DI#727 and #15974 (PR: #16311)

to be continued ...