They had no limiter back in '06... Only the engines started to desintegrate at those rpms, so the teams adjusted accordingly... BMW broke 21,000 late in '06 I think...
Several teams are rumored to have been reaching 22,000RPM very late in the season. It always made sense even with the coming homologation rules, since an engine that doesn't fall apart at 22,000RPM is also less likely to fall apart at slower speeds...