Gnome’s default file explorer does allow drag and drop to add a directory (folder) to the left sidebar. It just has to go into the middle list on the left sidebar. You cannot add things to the top list by drag and drop.
KDE is far far more customisable than Gnome, but changing too many settings can sometimes make KDE unstable. If you choose KDE, try changing only a few settings at a time. Gnome takes the approach of making it more beginner friendly by making it harder to break and providing an experience that Gnome devs think would be the best.
Gnome’s default file explorer does allow drag and drop to add a directory (folder) to the left sidebar. It just has to go into the middle list on the left sidebar. You cannot add things to the top list by drag and drop. KDE is far far more customisable than Gnome, but changing too many settings can sometimes make KDE unstable. If you choose KDE, try changing only a few settings at a time. Gnome takes the approach of making it more beginner friendly by making it harder to break and providing an experience that Gnome devs think would be the best.