The ncurses sources include the tmux and tmux-256color definitions, see
https://www.invisible-island.net/ncurses/terminfo.src.html#toc-_Tmux

No packages on Tumbleweed provide these definitions. Perhaps they can be added
to terminfo-base. Or a separate terminfo-tmux package (similar to the existing
terminfo-iterm and terminfo-screen) can be added.

A final possible solution would be to create a terminfo-extra package which
ships *everything else* that's not included in the existing terminfo packages.
There are so many missing definitions that can't be installed.

A workaround is to extract the definition from another system that includes it,
and then use /usr/bin/tic from ncurses-devel to install it locally. Flatpak
users can use something like:

    flatpak enter org.mozilla.firefox infocmp tmux-256color | tic -

The flatpak container ships with the needed definitions.