I am upgrading a Solr 4 server to Solr 8, and one issue I found is the format of solr.xml changed and I can no longer specify <cores defaultCoreName='collection1'>... - this leads to having to specify the core name in the URL's to search, dataimport etc. when in the old SOLR 4 the users didn't have to.
Is there a way to specify the default core in SOLR 8 for the purposes of defaulting the URL's not having that core to defaulting to it?
No, the concept of a "default core" has been removed, as early as Solr 5.
There are no longer direct references to a default core setting, but you can still come across some ancient texts hinting at its removal:
...the concept of default update URL is almost gone (Apache JIRA)