We deleted a custom field in Salesforce and then we got this error:
Salesforce Sync Error
Why: The following required fields are hidden from the Salesforce sync account firstname.lastname@example.org:
What to do now?
Edit the visibility permissions in Salesforce for the required fields.
Learn about required Salesforce permissions for the Salesforce sync account.
How can we update the mapping within Marketo to make sure that that field is no longer required to be mapped?
Open a case with support and have them remove them delete the field on the Marketo side. Only Marketo support can remove fields if you don't need that field any more.
This is apparently an opportunity field. This means you cannot hide it by yourself.
But normally, the sync should continue to work.
You may want to check that the field is not used somewhere (as a constraint in a smart list, typically).
Otherwise, as said, ask support to deep hide it (it's not completely removed, but at least no one will be able to see in Marketo).