This 'bug' still exists because it's not a bug.
[way more spoilers ahead]
There's the book 'How to handle Tenebrium' in Sacred Stone to learn Tenebrium for the whole party if you don't do Brandon's quest.
Beside that, there are items that give +1 Tenebrium, but that requires some drop luck of course.
The ability is only required to wield tenebrium weapons and does no longer increase damage of tenebrium weapons in EE.
And finally, there is exactly one single place in the game, where tenebrium weapons are required, to destroy the big blood stone in the lava area, because it is immune to any other damage type. Nothing else requires tenebrium weapons. If you unlocked all rooms in the Homestead before the First Garden, the big bloodstone might not even have to be destroyed if you can live with the Homestead being constantly fireballed.
The legend that tenebrium weapons are absolutely required stems from classic, when the tenebrium ability increased the damage of tenebrium weapons. It was not true back then and is still not true that one is 'screwed' without tenebrium weapons and without doing Brandon's quest.
Since tenebrium weapons can not be improved with crafting or blacksmithing, they are even inferior to normal drops or crafted weapons damage-wise in the Extended Edition, they might have been ahead in classic. But they were always only required in one single place.
If you have Bairdotr in the party, get her any tenebrium weapon (e.g. from the blacksmith apprentice in Sacred Stone) to destroy the bloodstone and after that you're done with most 'tenebrium problems'. Or steal the book from Lady Attenberah and learn Tenebrium for yourself and your preferred companions. At least for damaging, you never need tenebrium again. (I could not find any character entry with a negative 'shadow resistance', so there are not even mobs with a specific weakness to tenebrium weapons. Only one single item, the mentioned blood stone, requires tenebrium.)
The only thing the ability is good for after destroying the blood stone is to prevent getting the Rot when you pick up tenebrium weapons or the tenebrium bars that the death knights drop.
(The exception are shadow wands. Although they require the tenebrium ability to equip, Larian forgot to add them to the items that give Rot. THAT is what would be called a bug. Even when having Rot, the game can be finished although it might get annoying to have to heal that 1 HP damage per turn or per every 6 seconds out of combat.)
As said above, there is an alternative way to learn Tenebrium, there's not only Brandon's quest.
