Is the note following a tied note usually legato (for that same voice) in bach? this is the F♯ minor prelude from the wtc II BWV 883 for example measure 11 in the bass voice there is an F which is tied across the bar line. Would I slur or play the G which immediately follows as legato and then detach the A and the F which follow in that voice?
A "tie across a barline" is just an artefact of the way musical rhythms are written. Really, there is only one note that is 1.5 beats long, except that in conventional modern notation you can't write it with only one symbol.
The tie doesn't have any significance for whether the note is legato or not. Bach very rarely notated phrasing, though some editors have added their own ideas in published editions. None of the dynamics, tenuto marks, etc in your example were written by Bach himself.
In fact at some periods in the history of music, that tied F# would have been written as a single dotted quarter note, with the dot printed at the right hand side of the bar line where the tied note is in your score. If you look at first editions of Beethoven (for example on IMSLP) you will find examples of that notation.