Domain functional level requirement for 2015 on-prems

Hi

I would like to install CRM 2015 in my current domain (2003 native mode) but I'm not sure if this will work because of the domain functional level requirement for 2015 (Windows Server 2008 and above).

My deployment will not be an IFD and not use ADFS, nor will it use any time of client certificate (or even https for that matter).

Is the domain functional mode still a hard requirement in this scenario?

Thanks

Nicolas