Is it necessary for a UX designer to have front-end programming knowledge to give technically feasible and maintainable designs??

Ohh yes, UX is not plain design anymore.

@existas probably the people I work with are frustrating. They think i am lazy to code whenever I suggest a simpler way. :(

Imo, a UX designer should be able to code. Working as a full stack webdeveloper, I often had the UI/UX guy come over after finishing a story point and telling me to move this thing tjere and add that css property to that thing and after seeing it on the website, he saw, that it needed more whitespace and so on.

Tl;dr: yes!

I believe that every single person on this planet should know basic html/css so, yes.