It means you can't ask for any support from Esri after June 1, so do plan to upgrade to 10.9.1 which is supported until the end of 2027. In general, never stay on version x.0 once a x.1 or bigger is released. x.0 versions have the most bugs.
ArcGIS Enterprise Life Cycle | Esri Support
Once you upgrade to 10.9.1 you can install ArcMap on that server, but you must intall ArcMap version 10.8.2. 10.8.1 will not work.
GISP