Gimp 2.10 has unified the sharpen and unsharp mask filters that were two separate filters in earlier versions (both located in the Filters > Enhance submenu).
I can imagine that from an "engineering point of view" the move seemed to make sense, because they worked using a similar concept. However the new unified filter is not without disappointments. Allow me to describe:
The sharpen filter in v2.8
I found that sharpen worked well only for relative smallish images, like below ~ 2000px longer side, or so, and when cranked up to a high value, it provided results some may consider quite rough, not fit for professional publishing.
But I'm not a professional publisher. What I needed it for was sharpening (often low resolution and quality) images of various machinery and technical products, in order to observe and understand how these complex structures worked.
For this purpose it did not matter if the result looked a bit rough.
The unsharp mask filter in v2.8
The way I used this filter was a bit comparable to how the Clarify effect/filter worked in some earlier version of PaintShop Pro that I had the opportunity to try out. This clarify effect differentiated separate areas in a picture, often, for example, subject versus background, or overlapping objects, and made them stand apart more (so the eye perceived the spatial "depth" much easier, in effect, in an instant).
I used Gimp 2.8's unsharp mask filter to get somewhat comparable results by setting its Radius to around 100px and even above (combined with only a mild value for Amount).
I often used this trick also for images of landscapes.
The "unified" Sharpen (Unsharp Mask) filter in v2.10
Sharpening seems to have lost some of its edge: like it now prioritizes publishing-industry oriented quality over achieving extreme visual revealing power.
My unsharp mask trick for large areas now delivers unacceptable results, because it deposits so much dark into smaller-scale, midtone-but-in-a-brighter-context areas.
In the new, unified filter, increasing the Radius increases not only the distribution of the effect — the area that is impacted — but also the intensity: darks get even darker. This should however be be the responsibility of the Amount slider.
So now, if I reduce the Amount, the results do not seem to get applied evenly: even if I tone down the effect to be barely noticeable in most of the image, the darkness of small pockets persists disproportionally: crevices and small shadowy areas remain to stand out painfully dark and ugly from the rest of the image.
I never had such a challenge with the old Unsharp-mask filter.
Can I get back the old filters?
Do some plugins exist that would bring back the algorithms of the old filters? Or what other action can I take to benefit from the effects of the older variants?
I use Ubuntu 20.04 (in case it matters), but if you know something specific to Windows 10, I'd be interested to hear that too.
Following up on @BillyKerr's answer, I dug deep enough to find out that on Ubuntu 20.04, the standard available Gimp version is 2.10.18, this is the one I was using.
Now I have installed the "flatpak" variant too (it's like "Portable Apps" for Linux), which happens to be newer: 2.10.24.
In 2.10.24, an addition is present: the Blending options section is now available in the filter window, and digging in it, there is a choice between Default and Legacy. This looks exciting, but to be honest, in the first quick experiments I could not spot the difference between them. Anyways, I expect, if this would work, then this would offer a relief only for the Unsharp Mask challenge, as the new unified filter seems to be based on that.