It seems this column in ibf_members table is obsolete, I warned member, but column was not updated, so could be removed.

Last warn date is in ibf_members_warn_logs table.

I believe it's there for legacy reasons. As of 3.3 everything regarding warns was moved to another table as far as I know.

Yes it's possible.

I have deleted all old warnings and all members started with clear shield because a new warning system is pretty improved.