Hello. Why glibmm package was presented in old openwrt repo but dissappeared from last releases and lede?
Likely because nobody has volunteered to maintain it in the packages repo.
In 2014 there was a cleanup and switch to Github repo. All packages need a defined "maintainer" who takes care of the version updates etc. Starting in June 2014 packages were moved from svn.openwrt.org to Github, and only those packages were moved that somebody wants to maintain.
https://lists.openwrt.org/pipermail/openwrt-devel/2014-June/025810.html
https://forum.openwrt.org/viewtopic.php?id=52219
That glibmm package is still in the abandoned "oldpackages" repo, where those old package remain that have no active maintainer.
http://git.openwrt.org/?p=packages.git;a=summary