Marketo has been communicating this architectural change to its customers worldwide since August of 2016, yet only lately it has become clearer that the change will affect more than a few fields. Specifically this is true for Marketo customers whose Salesforce CRM edition is the Professional edition.
Tip: if you're unsure which edition your Salesforce org is running on, log in to your account there and simply hover the cursor on the browser tab where Salesforce loaded. The tab tooltip will show you the edition.
If you're not a Salesforce Pro user, you can stop reading here; the change won't affect you beyond the 16 fields that you were asked by Marketo to recreate in Salesforce (see here
, Marketo login required).