In MediaWiki 1.35 freshly downloaded from https://releases.wikimedia.org/mediawiki/1.35/mediawiki-1.35.9.tar.gz after calling composer update/composer install (Composer 1.10.26; I guess, MediaWiki 1.35 is not yet compatible with Composer 2), I get:

Installing wikimedia/avro (v1.9.0): Downloading (failed)    Failed to download wikimedia/avro from dist: The "https://api.github.com/repos/wikimedia/avro-php/zipball/b2e0c9d750da03d95ba979215397f62a6121ddea" file could not be downloaded (HTTP/1.1 404 Not Found)

and composer update/composer install fails.

The repository is gone from GitHub.

I presume the tarball already has all dependencies installed; yet it can be an indication of some other problem.