When substate regional estimates are produced, are the regions the same each time?

No. Regions are defined for NSDUH by state governments and may differ between years. For each set of estimates, there is a report with definitions and maps for that set, as well as shapefiles that can be used in geostatistical software. 

Because substate estimates are not available for 2018-2020, regional definitions for those years can be found in Appendix I of the 2023 Restricted-Use File Codebook, available on the Research Data Center page.