Started looking into this and additional functionality from IRE and came across your post. A couple of things based on below. I believe the custom property for a Robust Import Set Transformer where you set Properties like in your second screenshot, is actually ire.skip_updating_last_scan_to_now. Also this property per docs won't help you with most recent discovery field (last_discovered) or discovery_source.
In docs the only thing I see that supports the idea of preventing something going through IRE from updating most recent discovery is "glide.identification_engine.skip_updating_source_last_discovered_if_older". Everything else I've found calls outlast_scan field on the sys_object_source table.
So I think this is the one you want
glide.identification_engine.skip_updating_source_last_discovered_if_older
Determines how IRE updates the last_discovered and the discovery_source attributes in the CMDB.
TRUE: If last_discovered is provided in the payload and it is older than the last_discovered of the CI in the CMDB, IRE does not use the payload values to update the last_discovered and the discovery_source attributes in the CMDB.
FALSE: Even if the last_discovered provided in the payload is older than the last_discovered of the CI in the CMDB, IRE uses the payload values to update the last_discovered and the discovery_source attributes in the CMDB.
This is great, but it doesn't sound like it guarantees that your other sources WON'T set last_discovered because a source might have scanned data that is more current than say discovery, depending on how often you are running discovery.
Maybe related:
I also see glide.identification_engine.batch_update_last_discovered, which says:
Controls batch update of last_discovered field in CIs that are being processed by the identification engine.
Set to false if there are business rules that apply to the last_discovered field, and you want to trigger these rules when calling an Identification and Reconciliation API.
Honestly I don't know what this is trying to convey. If you have BRs that will set last_discovered, set this property to false so IRE doesn't also try to set it in batches? What if you set it to true? I assume it doesn't prevent BRs from running, so does it allow the BR to set last_discovered AND IRE batch updates will as well?
EDIT: also check out glide.identification_engine.ire_message_listener_skip_updating_source_last_discovered_to_now
