I find that drafting by hand has been lost, and the beautiful nuances typical of each draftsman.
Now all drawings are done with programs such as AutoCAD, easy to modify but dead as far as presentation

semaphore? reading and making maps, I can classify plant communities from high altitude stereo photographs, and I can do so without the lenses, you just have to train your eyes.
For me though, the greatest loss is in the everyday items. How many people can bake a loaf of bread using the basics and without following a recipe? How many people in retail could order stock from another country without a computer, orders, bills of lading, letters of credit, remissions? Who knows shipping terms for stock? Such are not taught now, few people have to deal with them, I was taught all of these at school.

Things used to get Fixed. The beauty and utility of the transistor radio. A rite of passage.

Adjusting the "rabbit ears" to improve TV reception.

Seamstress, shoerepair, shoeshine, many honest living gone with the times. I need to resole some shoes myself! The Beauty of Cursive Writing.

In the Marines I was a microwave radio technician. I am sure they have long gone by the wayside as have many other things. Remember cartoons and newsreels in the movies and double features. Then there was the mangle for ironing A great machine.Comics cost .10 cents and there still was penny candy.Bread cost $.20 and two weeks of grocery's cost $40.00.So many things have disappeared including manners and consideration for others.

-My father and paternal Grandfathers were both carpenters from them I learned to work wood, repair things, basic carpentry, glazing and electrics, my uncle was a auto mechanic from him I learned car and machine maintenance.
My maternal grandfather was a merchant seaman, as was his brother, from them I learned organisation, packing, sewing, navigation, geography, knots.
Growing up I learned skills that have saved me a fortune in repair bills, replacements bill and an aptitude for learning useful tings.
When I married and found that my in laws would call an electrician to change a light bulb or a plug, I was to coin a phrase gob smacked. When I found people who would pay literally hundreds of pounds to fix a mechanical fault in a car that was only a moment's work with a spanner, I thought they were joking. Then when I find my local mechanic charging thousands to replace an entire engine block, when all that is needed is for a set of spark plugs to be replaced and the client happily pays it because they know no better I am shocked.
Today when I find my grandchildren are not taught mental arithmetic but instead how to use a computer or calculator correctly, I weep for the level of techno reliance, the level of use, throw away and replace consumerism and the sheer depths to which technical know how in the average person has deteriorated or is non existent I fear for the level of induced helplessness most will suffer should the power ever go out for a prolonged term.

The use of slide rules in engineering and math.

The ability to maintain and repair analog circuitry.

There are also skills that, while not quite dead, are on life support. Maintaining pipe organs, particularly mechanical-action instruments. The maintenance of automobiles that don't have computer or IC-controlled functions (distributor caps, points, etc; my father, who was an auto mechanic in the 1930s and 40s and an aircraft mechanic after that, looked under his hood in the 1990s, shook his head, and closed it up and called the garage; he didn't recognize most of what was in there anymore).

There are skills that are very much alive but relegated to narrow niches of which most technicians are blissfully unaware. Assembly language programming. Reading hexadecimal diagnostic core dumps. Increasingly, a theoretical understanding of data structures and sorting algorithms and the like is considered optional. Someone, somewhere down under the hood of whatever operating system or API you're using, has to understand that stuff and maintain it; but most junior programmers just call a Sort() method and don't even care how it's done, so long as it's accurate and fast enough. And they look that up online at Stack Overflow or with Google; they are hooking together other people's recipies without really understanding the ingredients or how the "tinker toys" interact.

A lot of this just reflects that you have to know a heck of a lot to have a deep understanding of what underpins the machinery of modern existence, and it's not practical to be intimate with it all. But it's useful to delve deeply once in awhile. For example I once had a weird bug in some code I wrote and after satisfying myself that I was doing everything right, I got hold of the library source code and traced down into it and figured out what Microsoft had done wrong (they had forgot to un-comment something they'd commented out for testing) and how to work around it. I don't think 1 in a 100 software developers are actually capable of that anymore, which is why I command top dollar. Not that I'm particularly brilliant -- far from it -- I'm just particularly curious and passionate about my work. Many times I look over the shoulder of someone working for or with me, who has been tearing their hair out for hours, and immediately see the problem and the solution they are blind to. That is experience and deep knowledge combined with trained intuition. Everyone should have some aspect of life that they have that deep understanding of, in my view. It's satisfying. Sadly, fewer and fewer of us even get the chance to experience that.

Although I never mastered calculus, or higher mathematics, my grasp of the basic functions is quite good. When proferring cash payment, at a retail establishment, for instance, I can usually figure out my change before the register does so. I can also figure decimal values and percentages fairly quickly and accurately. These are basic skills, but they often come in handy, such as when working with wood, or converting SAE to metric, etc.

I was in Quality Assurance for many years and these skills were quite necessary.

Sadly, I don't think skills really become lost unless they are really not needed any more. Modern communications systems have obviated the need for a lot of the old systems, and if we need to revive them, through catastrophe, they will not be too hard to learn again.