Is it possible to set event retention as per severity of events?
Can OK and INFO severity events have lower event retention period configured?

It is not possible to set event retention based on severity of events in any cell version, Event retention config is available only for following two status of event.

1. CLOSED
2. OPEN

EventDBKeepClosed and EventDBKeepNonClosed properties in mcell.conf file at location $MCELL_HOME/etc/<cellname> Control it.

Detailed clean up process at cell level is documented in BMC Online document - https://docs.bmc.com/docs/TSInfrastructure11304/event-repository-cleanup-parameters-937359485.html

By Default, CLOSED event stays in mcdb of cell for 3 days that is much lower than OPEN event (default 30 day) .

Though we don’t have any severity-based event retention configuration in product, one can consider to use mc_timeout slot value (Using Timeout Policy in Administration Console or MRL code) on low severity events.  BMC recommend to use mc_timeout slot values selectively for known events such as OK severity events from SNMP event adapters, Network Heartbeat events etc.

Please do not set timeout on ALARM and ABNORMALITY events as they are generated by RATE process of TSIM. We store RATE generated events into TSIM DB table and that records must be in sync with MCDB of cell.  Setting mc_timeout will not clear entry of events into TSIM DB table as a result subsequent ALARM or ABNORMALITY events will be missed