so lets say my opponent has the new teferi in play
i got God-Eternal Kefnet and reveal an instant or sorcery. will i be allowed to cast the copy or not ?
No. Since the stack is not empty (Kefnet's trigger is still resolving) that means it is a time when you can't normally cast a Sorcery. As such, Teferi will prevent you from casting the spell. In Magic "can't" beats "can" (or, to put it another way, a restriction will override a permission) and since Teferi restricts when you can cast spells, it doesn't matter that Kefnet is permitting it.
For the purposes of the first ability of Teferi, Time Raveler, a copy of a card that's cast is also a spell (C.R. 112.1b, 706.12). If an opponent of yours controls Teferi, you can't cast any spell, including a copy of a card, during the resolution of Kefnet's second ability, even if it's your main phase, since the Kefnet ability would still be on the stack at that time (C.R. 117.4, 608.2c, 307.5, 101.2; see also C.R. 117.1a, 601.3, 608.2g).
EDIT (Jul. 16): Edited.
EDIT (Sep. 13): Edited, including because some rules were renumbered with Core Set 2020.
EDIT (Sep. 14): Edited.
EDIT (Nov. 18, 2021): Edited to conform to rule renumberings.
i got God-Eternal Kefnet and reveal an instant or sorcery. will i be allowed to cast the copy or not ?
EDIT (Jul. 16): Edited.
EDIT (Sep. 13): Edited, including because some rules were renumbered with Core Set 2020.
EDIT (Sep. 14): Edited.
EDIT (Nov. 18, 2021): Edited to conform to rule renumberings.