If you are using the 8.x-4.x version of the Search API solr module with Hosted Apache Solr, you may see a warning like the following in your Drupal site's 'Status report' for your configured Solr Server:

Schema modified
Your config-set contains manually added customizations. Be aware that these will be lost when the config-set needs to be regenerated.

The reason for this is Hosted Apache Solr automatically adds a few small modifications to file paths in the configuration so the generated Search API Solr configuration works correctly with Hosted Apache Solr's servers.

You should not worry about this warning, as long as search and indexing work correctly, there is nothing to worry about.