Especially in keyboard Style, I am left with very few options for voice leading if I try to prepare and resolve the dissonant 7th of the super tonic and sub-dominant chords in the same voice. How important is this Rule and how much should I care about it in the 21st century? I can imagine if it is a chorale or something where each person needs to sing one line but in other genres does it make a difference?
The rules you are learning are practical, not aesthetic. That is, following them describes what a certain era of composers did in their practice; it's not a formula for producing one's own music, which can have any sound and follow any rules one likes.
The "rules" — such as preparing dissonances in the same voice — were developed several hundred years ago, with sounding like Palestrina as the general guide. If one wants to compose music that sounds like Palestrina, then the rules should be followed. If not, all bets are off.
As the rules developed, they encompassed composers like Bach, Mozart, Beethoven, etc...., up until the 20th century (roughly).
21st-century classical music frequently throws away all of "the rules" and is based on entirely different models for music composition. Jazz and pop prioritize other concerns than smooth voice leading.
The harmony exercises you have been showing us are intended, I think, to be worked in Bach-style 4-voice texture, laid out as SATB. In this context, where 4 independent voices weave together to form the harmony, voice leading is important.
Sometimes, however, you choose instead to write in 'keyboard style'. Block chords in the right hand, a bass line in the left. Yes, in this style voice-leading often goes out the window!