Can I make a complaint if my financial service provider is de-registered from the FSPR, or FDRS membership has terminated?

Quite likely.

You can make a complaint to FDRS about a financial service provider who has been de registered by the Financial Service Providers Register, or has been terminated by FDRS (see Search for Scheme Members)  so long as the issue that gave rise to the complaint occurred after 1 October 2010 and whilst the financial service provider (or scheme member) was a member of FDRS.