Found this issue for float topic is no more working with the new Lithosphere Design.
I have added post to "float to top" from options menu but it still displays on the same order as it was earlier in discussion board.
See screenshot attached:
OOB "message-list" have that control for the messages.
I Guess, The current new design is customized now in community which does not support this feature.
Thanks for flagging @Parshant looks like global floating is working but not the local float. Will get it looked at.