During my perusal of the forum I have read that several of the classic card books present an outdated style of writing and patter. From my own readings I can understand how one may arrive at these conclusions. However, I just read some comments regarding an older card book (which I unfortunately forgot the name of) that opined that some of the actual techniques taught to achieve particular sleights were outdated. I believe one of the referenced sleights was the side steal but I am not 100% certain. No matter, however I am curious as to how an actual card-handling technique based on manual manipulation can be considered outdated. Is it simply that someone develops a faster or more direct method of doing the same move?

I'm relatively new to this so I was wondering if those with more experience and who are well-read in this art may have a perspective on how a technique becomes outdated.

In my own opinion as a worker on a regular basis, I don't feel any move is outdated, rather as you mention streamlined for one's own individual abilities or limitations. Some people just can't, for whatever reason perform a classis pass or a one handed top palm for example. Some feel more comfortable with a double undercut or top palm with the aid of the other hand.

I strongly believe that if you want the most out of this art to practice and become familiar with many moves. They become crucial imo, in different scenarios like when you make a mistake, lose a card or go in a different direction than you had planned.

Here is a appreviated list of the sleights I use on a regular basis:
Classic Pass, KM move, JAPH maneuver, One Handed Top Palm, Double Undercuts, Thump counts, Pinky breaks, Double and Triple Turnovers etc.

It depends on YOU and where you want to go with card magic. ( not the question you asked but I felt important to add)

I don't find that the teaching of technique is dated, they teach step by step and the result is the same as far as I'm concerned. I learn a lot from old books. The language can certainly be dated. However to me all that actually means is BETTER USE OF THE RANGE OF THE ENGLISH LANGUAGE! (Sorry, got carried away shouting there.)
I know, not really answering the question but hopefully I've added something of value.
Paul