(In reply to José M. Muñoz from comment #19)
Yes, being able to force a particular charset was useful, but somehow more an more niche features are removed from Thunderbird. You can now use the add-on I mentioned to do it.
I would say that ability to deal with partially broken messages has a real value for a mail user agent. It is rarely required last years, but still...
Concerning extension, for browsers WebExtensions API limits to some extent what an add-on can do. Charset menu in Thunderbird requires experiments API, so full access. As the consequence, the author should have ideal reputation, e.g. it is necessary to be sure that he will not accidentally lost control, will not sell the extension to untrusted 3rd party company, etc. This particular extension does not have a link to source code repository, no license specified (though GPL-3 inside .xpi). I do not think, it is comfortable situation when someone is in a hurry to read a message, but it is necessary to install a suspicious add-on.
I have tried charset detection for several messages having KOI8-R and windows-1251 encodings. Charset is recognized correctly for these examples. KOI8-U instead of KOI8-R is not a real problem since messages do not have characters in the range where encodings differ from each other.