Fiscal Years and Periods have been replaced with a single setting "Fiscal Year Last Day" which can be found in Accounting > Configuration > Settings. So if "Fiscal Year Last Day" is set to dec 31, then each jan 1st a new Fiscal Year would start.
On a purchase invoice, the Fiscal Period field was replaced with an "Accounting Date" field, which can be used to control which Fiscal year the document is booked in.
I have not seen any explanation as to why this was done. I imagine it's meant as a simplification, and it certainly adds a degree of flexibility, although I also see a some issues with this.