There's no information on develop.kde.org about channels which can be used to communicate with developers of KDE (think "community" section).
Actually it's not easy to find this information at kde.org, either. One has to scroll all the way down, where there are Products, Develop, News & Press, Resources and Destinations sections, find "Community Wiki" link in Resources section (among several other topics in all sections), go there, then click "Welcome to KDE" (again, one of a few topics) and then find Chat paragraph with information on Matrix. I suggest placing this information in much more prominent way like many other projects nowadays do.