Dear Martin,
The master branch of OpenFAST includes linearization for land-based and floating offshore turbines, with the later pertaining only to rigid-body modeling of the substructure. Modules that support linearization include ElastoDyn, BeamDyn, InflowWind, AeroDyn, ServoDyn, HydroDyn, and MAP++.
Recently, we’ve been working to upgrade OpenFAST to model floating substructures as flexible bodies with member-level load calculations, including upgrades to SubDyn, HydroDyn, and the OpenFAST glue code. Along with that upgrade is enhanced linearization that also includes support for linearization of SubDyn, which then means the OpenFAST linearization will support flexible support structures, both fixed bottom and floating. More information is available on this forum here: Extending the dimensions of matrices related to equation of motion - #11 by Jason.Jonkman.
I hope that helps.
Best regards,