Hardly ever? We never use 'ruth'. It's archaic. Though we do still use 'rue'. which is related to it.
We don't use 'feck' to mean 'effect','value' or 'worth' either ('What the feck?' is not an enquiry aboiut worth). We do use 'feckless' though.
There are other examples of words formed from roots which are now obsolete or archaic. An unusual one is unkempt, which meant 'uncombed'. 'Kempt' was the past participle of 'kemb' a dialect word meaning ' to comb', it wasn't used in regular English but unkempt has always been.



