I am working on adding new scores, but in many cases these old things are either the only editions published or the only editions we can take.
IMSLP accepts urtext editions over 25 years old. We have a good amount of these from series for the most famous composers (e.g., Mozart, Bach...) but these are only a minority of the collection. We of course would like to add more.
Otherwise, for "original" editorial editions, as well as arrangements, etc., we take them so long as the contributors of original creative contributors died in 1971 or earlier, or, as the case may be with amateur editions, with permission (note: we actually do have a significant number of quite good editions/arrangements contributed with permission, but we have a lot of files on the site overall).
The other thing is, since we draw from many existing digitizations (such as national libraries), and most of their digitized documents are quite old.
If you have any particular requests, please add them to the wishlist: https://imslp.org/wiki/Wishlist
Anyway, we are not really like Wikipedia, since we have a great deal of those primary sources on our site (incl. many autograph mss., as well as historical editions), and we're fundamentally more a collection of sources than a synthesis of them like WP. But we are certainly not a complete collection of sources, by any means (for both practical and legal reasons).
Do you have a working definition of "urtext"? A lot of famous Bach and Mozart keyboard stuff was written on C clef for the right hand, but many so-called urtexts transcribe it to G. I don't understand how this can be seriously called urtext.
I feel very uneasy changing the notation of the original, even if the music sounds the same, and naming it "urtext".
No big deal... after all, you can buy "urtext" with editor-provided fingerings from many publishers, so the word is almost meaningless at this point.
Maybe you don't like modern clefs, but by no means are they any sort of "original" work by the standards of copyright law in any country. (Even in the small handful of countries that recognize typographical arrangements/engravings per se as copyrightable in themselves, like the UK, changing the clefs used is not considered original in any way.)
Pure transposition, by the way, is also not considered an original copyrightable creative change.
I think you have focused in on the wrong aspect entirely from the musical standpoint... the purpose of any of these editions is not to be a facsimile of some existing document (facsimiles of autograph manuscripts exist), but really just to be critical editions taking into account a number of sources and intending to establish the work as it is in a certain state (generally, an early state) according to certain editorial principles, rather than reflecting the editor's personal creative opinions on how the work should be interpreted.
As for the fingerings provided in some critical editions, these are not really relevant, because they are clearly marked (when applicable) as separate from the text itself, and they can easily be redacted (as we need to do sometimes) or ignored. By the same token, the preface or editorial footnotes in these editions are separate from the presented musical object — do you believe a preface or footnotes somehow makes the thing provided in the same pages indescribable?
You picked my interest.
- what makes an "edition" ("score"?) outdated?
- can you give examples for a weird editorial decision, maybe by additionally providing an alternative good editorial version?
- what happened that something deemed correct (?) over 50 years ago is nowadays something no one agrees with?
That said imslp is still a life-saver and playing music would be a lot more painful (and expensive!) without it
https://imslp.org/wiki/IMSLP:Copyright_Made_Simple#Urtext_Ed...
(copyright laws afford Urtext editions shorter terms of protection than original works)
Compare that with: https://imslp.org/wiki/File:PMLP05948-WTC_1_No_1-12.pdf
A modern Urtext edition would also include detailed information about which manuscript and early edition sources it was prepared from, and any unresolved or variant readings between these sources, with performance suggestions (apart sometimes from fingerings) relegated to supplementary notes that are clearly written by the editors and not the composer.