This addon hasn’t been officially updated for some time now and has been broken by newer versions of Blender. I have downloaded oRazeD’s unofficial update (https://github.com/oRazeD/UVToolkit) and will be testing it. But I suspect that soon one of my most essential UV addons will become permanently unavailable.
The great thing about UV Toolkit is that it rolled together so many useful features into one menu. It may take more than one addon to replace it.
Can anyone suggest a good replacement?
UV Toolkit is being maintained by other developer on extensions platform
@nickberckley You linked to same fork as OP 
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@sboerner
As far as i tested this fork have everything working, and i guess that if current maintainers give up with maintaining it, someone else will pick it up, no need to panic yet.
and free:
and Textools have a lot of similarities:
Good to know – thanks, everyone. I also have TexTools installed and use it alongside UVToolkit. Guess this means I can finally upgrade to 4.2.
I now use ZenUV (+ UVPackmaster3 for layout/packing) and feel like it is the best overall UV support Addon out there: it provides a superset of what UVToolkit did.
ZenUV is actively developed and well supported, with good documentation, YouTubes, etc.
That being said, I did use UVToolkit previously and it was an excellent extension to the native Blender tools and TexTools. I did not know UVToolkit was still alive, but do see it in Available Extensions.
If you are tempted by ZenUV, take a look at their Bundle, which includes another excellent Addon, ZenSets.
One more addon to the list: