1)http://groups-beta.google.com/group/comp.lang.scheme/msg/b7075e4ca751dbdb

is now

https://groups.google.com/forum/#!msg/comp.lang.scheme/9x-1PRs6dMk/29tRp0xeB7cJ

2)

http://groups-beta.google.com/group/comp.lang.scheme/msg/180c7627853c288e

is now

https://groups.google.com/forum/#!msg/comp.lang.scheme/9x-1PRs6dMk/jig8hSd2DBgJ

3)Matthew Flatt - Composable and Compilable Macros You Want it When?
is now
https://www.cs.utah.edu/plt/publications/macromod.pdf

4)R. Kent Dybvig - Schez Scheme user's guide
is now
https://cisco.github.io/ChezScheme/csug9.5/csug.html
But since it is version-dependent, perhaps is is better to give link here:
https://cisco.github.io/ChezScheme/#docs

5)Robert Hieb, R. Kent Dybvig and Carl Bruggeman
    - Syntactic Abstraction in Scheme
is now
https://cs.indiana.edu/~dyb/pubs/LaSC-5-4-pp295-326.pdf

6)R. Kent Dybvig - Writing hygienic macros in syntax-case
is now
https://doc.lagout.org/programmation/Lisp/Scheme/Programming%20With%20Hygienic%20Macros%20-%20R%20K%20Dybvig.pdf

7)http://library.readscheme.org/page3.html
is dead

8)William D. Clinger - Hygienic macros through explicit renaming.
is now
https://3e8.org/pub/scheme/doc/lisp-pointers/v4i4/p25-clinger.pdf

9)Eugene E. Kohlbecker, Daniel P. Friedman, Matthias Felleisen and
Bruce F. Duba    - Hygienic macro expansion
is now
https://web.cs.ucdavis.edu/~devanbu/teaching/260/kohlbecker.pdf

10)Alan Bawden - Quasiquotation in Lisp
is now
https://pdfs.semanticscholar.org/0bc4/5fe8b371c53b70ee4aa11615111cd7c4ef03.pdf

11)(Robert Hieb, R. Kent Dybvig - A compatible low-level macro facility)
R4RS can be found here:
http://people.csail.mit.edu/jaffer/r4rs.pdf

12)Matthew Flatt      - Introducing an Identifier into the Lexical
Context of a Macro Call
http://list.cs.brown.edu/pipermail/plt-scheme/2004-October/006891.html
can only be found as
http://web.archive.org/web/20060905080950/http://list.cs.brown.edu/pipermail/plt-scheme/2004-October/006891.html

14)Christian Queinnec - Macroexpansion Reflective Tower
https://christian.queinnec.org/PDF/towexp.pdf

--
Yours sincerely, Vladimir Nikishkin