Correct.
The underlying sticking point is the version of Lucene that is used.
AEM at its latest (6.5) uses
<lucene.version>4.7.1</lucene.version>
<solr.version>6.6.6</solr.version
In Apache Solr 7.x [1] there were major changes to the schema.xml, and even further changes in 8.x [2]
[1] - https://lucene.apache.org/solr/guide/7_6/major-changes-in-solr-7.html
[2] - 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 :
https://issues.apache.org/jira/browse/OAK-8235
The description on that JIRA with the Apache foundation describes in more detail :
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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.
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