Why there is no dedicated Linux (at least for Ubuntu) section/option/filter/sorting function in the dropdown topic menu at this site?
Dragonmouth is right. Although it doesn't appear in the "Topics" menu, we do have a Linux section which appears on our topics page and in other pages as relevant.
In the right column, under "Browse Questions", 7th line down, next to the word "laptop" in a small font there is a word "linux" in an even smaller font. Click on that and only Linux related Q & A will be displayed.
Also under "Top List" there is an option to "View All" Click on that and "Best of Linux" icon will be among the ones displayed.
As to why Linux was removed from "Topics" I can't tell you. Linux Topics got lost in the re-design of this site.