The current AEM documentation on helpx for MSRP configuration was all originally written in the AEM 6.2/6.1 era, and has not been updated since then. Namely https://helpx.adobe.com/experience-manager/6-5/communities/using/msrp.html
MSRP - MongoDB Storage Resource Provider (same document for all AEM versions).
Solr 4.10 and Solr 5 are multiple major versions out of date, as the latest Solr version is 8.2.0.
My two questions are:
(a) What is the latest version of Solr currently supported by Adobe for use in MSRP configurations?
(b) Where can one download the current solrconfig.xml and other configs, now that the links on the documentation no longer work?
The underlying sticking point is the version of Lucene that is used.
AEM at its latest (6.5) uses
In Apache Solr 7.x  there were major changes to the schema.xml, and even further changes in 8.x 
 - https://lucene.apache.org/solr/guide/7_6/major-changes-in-solr-7.html
 - https://lucene.apache.org/solr/guide/8_0/major-changes-in-solr-8.html
Since AEM is built on top of Apache Jackrabbit Oak and OAK uses such an older version of Solr and by extension, lucene -- we can't support newer versions of Solr due to non-backwards compatible schema changes.
Solr was upgraded to 6.6.6 here just a few months ago :
The description on that JIRA with the Apache foundation describes in more detail :
In order to support scenarios leveraging Solr as an Oak index, it is recommended to upgrade to a supported version.
Having Solr 8 being released just recently, we could upgrade to that or to 7.x, however I suggest to upgrade to Solr 6.6.6 which would require a more flawless update for people using 5.5.5, because 6.x is back compatible with 5.x.
Thanks for that, and that makes sense. For the application I'm building out though, we're only using Solr for MSRP for User-Generaged Content, and aren't using it as an Oak index, so I was hoping it wouldn't be bound by some of the same limitations. I see that there are solrconfig.xml and schema.xml files in 6.5 for Solr 7.x and 6.x, but not for 6.2 unfortunately.
It I think we'll just have to keep soldiering on with 5.5 for the moment, though we're still having issues going Solr standalone -> SolrCloud (nasty negativeArraySize exception that I've not yet gotten to the bottom of.)
Thanks. From everything I've been able to find, it does indeed look like nothing better than Solr 5.5 is currently supported on AEM, even on 6.5.