I have never measured a quilt seam allowance to see if if was "scant" or not. I piece on a straight stitch machine with a narrow foot, and normally I get good results with a tiny bit of fabric showing past the foot when I sew. I also like to sew really small stuff and often cut a bit large and then trim after I've got some pieces sewn together, so exact seam allowance does not matter.

I also like to cut the pieces I know will be on the outer edges of a block on the large size (the part that will extend) so when I square up, I can have an exact measure and not someothing that is shy of what it should be.

I also make sure the lines of my ruler are on the fabric, not at the edge. That gives me a thread or two more fabric so if my seam is a thread or two larger, it all works out. I have to be careful when I use my ruler with the slots (not remembering the name) as that cuts exact measures, and not the thread or two larger. But like others have said, what is in the seam allowance isn't the important part, it's what is the finished measure.

I've found that whether a pattern says "scant" or not - it's the same when you look at the cutting measures if it uses rotary cutting. A pattern that has you cut 2.5" square to make a finished 6" - 9 patch is going to be exactly the same whether is says to sew scant or not - because you have to sew the exact same seam allowance to get the end. For there to be a true difference, one would have to be cut differently than the other.

Apparently the Featherweight Shop has several feet available some for scant 1/4 inch and some for “true” 1/4 inch! They even have a video explaining them. Of course these are for Featherweights.
And yes, I ordered some!!

I don't like to hear "scant" 1/4". I have a 1/4" foot but I think it's a tad over 1/4" so I've tried scanting it up a bit. Every time I try to do a scat, seems like my block is too large. Then, I have to cut it down & generally find out I'm missing points on things like stars, etc. My eyesight is not good enough to see 1 or 2 threads on a piece of fabric so I can't go by threads. I'll just keep using my 1/4" foot & hope for the best.

The Featherweight Shop also have a seam guide with a scant and regular quarter inch seam allowance. It looks like it could be used on any machine. I have a generic one that I use to set all my machines when a scant 1/4" is called for. As was pointed out, it's only a couple threads less than a regular 1/4 inch and I've never had problems with them coming out.

I don't know about an 1/8" seam allowance. That's awfully small. I think I'd redo. Especially if it's something that is going to be washed a lot.

The whole idea of the scant 1/4" is that each seam takes up a few thread widths of a fabric piece because of the thickness of the fabric and thread at the seam. By sewing "scant" those few threads (as the Kimberley Einmo video shows), you don't lose any width of the finished pieces.

For example, say your blocks are supposed to finish 12" square and that some blocks are just a single piece of fabric, while others have 4 seams in them.

The solid piece is cut at 12.5" and stays 12.5" because it has no seams and no width loss due to seaming.

The block with 4 seams, if each is sewn as a full 1/4" vs scant, could be a total of up to 12-16 thread widths narrower than 12.5" after those seams are sewn. Now that block is smaller than the 12.5" block it needs to be joined to.

In a case where all the blocks have similar numbers of seams, you won't run into an issue. It's when you have different block piecing with different number of seams that you're trying to join that the "scant 1/4" " makes a big difference.

The first photo is a full seam allowance - note that you can see the edge of the fabric to the right of the presser foot but you cannot see the 1/4" mark on the throatplate because it is covered by the fabric.

The second photo is a scant seam - note that the edge of the fabric is just under the edge of the presser and you can see the 1/4" guide mark on the throat plate which I use to align my seam, keeping the fabric edge to the left of the mark.

I've learned that if it needs a " scant" 1/4", it's best if l simply cut one or 2 threads past the required measurements. I found when l stitch the scant quarter that some fabrics that are prone to ravelling have " fringing" very close to stitch line. I use many fabrics donated to our group for charity quilts. Even some of those that come from well known quilt fabric manufacturers can be ' thready'.

If my fabric is over/covering the 1/4 throatplate mark, like you showed in the picture, with my machine, my seam measures greater than 1/4".

I get exactly a 1/4" seam using my 1/4" pressure foot and with running the edge of my fabric along the 1/4" etched line on the throat plate as an edge guide.

It does depend on the machine. I have 4 machines that I use for piecing. I use a seam guide a lot because if I"m switching machines, I tend to forget how I get the seam allowance I like.

Your seams should not be 1/8 inch. A scant 1/4 is a thread or two less. If the pattern wanted 1/8 inch, they would have said that.
find the scant measurement and put painters tape on your machine. If you have needle adjustment, that works great too.