the RHDS-10 custom schema is in /etc/dirsrv/slapd-*/schema/99user.ldif
while the "core" schema files have now been located in /usr/share/dirsrv/schema/
you can till use the /etc/dirsrv/slapd-instance_name/schema/ directory , but see the caveat in the online doc at:
12.1.5. Schema Replication
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Schema files other than /etc/dirsrv/slapd-instance_name/schema/99user.ldif are used.
Directory Server enables you to add additional schema files in the /etc/dirsrv/slapd-instance_name/schema/ directory. However, only the CSN in the 99user.ldif file is updated. For this reasons, other schema file are only used locally and are not automatically transferred to replication partners. Copy the updated schema file manually to the consumers and reload the schema. For details, see Section 12.7, “Dynamically Reloading Schema”.
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that may be confusing when used to the previous schema files and location, and the reasons are documented in the upstream ticket:
https://pagure.io/389-ds-base/issue/48163 / If /etc/dirsrv/schema is version-specific, it should not live in /etc