As FB has rebranded itself to Meta, can anyone still register domains like Meta{keyword}.{anyext} or simply Meta.{anyext}? what a domainer should keep in mind?
As long as you have a use or defense in mind yes. Meta is such a generic term FB will have a difficult time arguing most registrations. The only exception imo is who ever registered Metabook.com that feels close enough a TM argument is a real risk.