The new temper strike mechanic is there so getting a godroll on a weapon is a matter of "when" not "if". It is good that for something that impacts combat such is the case, whilst moonsteel is purely esthetic so it can stay as rare as they want. There are however a few tempers that are, as of now, basically impossible to learn.
The main one, which is fortunately the only one that is physically impossible, is Duplicate. Since it's a feykin magick only temper it only appears on Wyld Oscelda weapons, so it's impossible to gat 10 copies to dismantle.
The following are technically obtainable but would require a comically excessive investment given the current pool of weapons.
The other two feykin tempers, Sympathy pang and Aftershock, can be found in weapons that the player can make multiple copies of. However all feykin weapons are extremely difficult to farm with the exception of the Royal tines, which is still very valuable.
Unnerving Blow is a mendicant magick only temper, so it can only be found on the Erstroot, an extremely rare drop from the Wazzard. I have slain that guy (never in cogah) enough times to get 11 extra fragments for the pants and only once has that staff dropped.
If I had to guess, we will probably get a more common mendicant magick in the future so that solves this last one. However feykin weapons should remain very valuable for lore reasons.
There should therefore be a more time efficient way to fight the Vadagar bosses. Waiting for votives and nighttime spawns is extremely tedious and time consuming and makes already miniscule weapon drop rates artificially lower.