How to install a package from an older version of TeXLive?

An archived version of TeX Live 2010's tlnet in its final state is kept on the TUG server. You can use it by doing

tlmgr option repository ftp://tug.org/historic/systems/texlive/2010/tlnet-final

then

tlmgr install collection-xetex

(for example) should work.

FYI, there are several mirrors of the historic repository (more are welcome), including some accessible via http(s), thus avoiding problems with ftp.

https://tug.org/historic/