As you may have heard, some grammatical rules have some inconsistent variants that we call "wavering" or 言葉の揺れ.
Originally Posted by Lisa Wang
In Japanese teaching, teachers or textbooks writers try to be consistent with one rule so as not to confuse the learners, but in real life, we cannot control them and accept such variants although some conservative people tend to whine about "misuse"s.
I am pretty sure you have learned to switch "を" into "が" when you make both potential and passive forms, but in real life, both are used for potentials.
・・・それを受け入れる+できる → それを受け入れることができる → それを受け入れられる can happen.
・・・それを受け入れる+できる → それを受け入れることができる → それが受け入れられる at JSL classes and normal writing.
・・・私がそれを受け入れる → それが（私に）受け入れられる always happens.
Also, You tend to find many mixed up を and が (as well as mixed up は and が) especially on internet, and also in game scripts and minor comics where no experts(language-wise) proofread.
Now with その約束を忘れられてはいない, this is a suffering passive, or some people call it "indirect passive". These passives take を instead of が, because the actor is the one who gets disturbed and the object is not considered a subject.
(私は）弟にケーキを食べられた。 I had my cake eaten by my brother.In this particular case, I hope I do not have the promise forgotten (by her). "I" am the one who would get disturbed by having the promise forgotten and we consider "I" as the recipient of the disturbance (or suffering) and 約束 does not take が as a subject maker.
（私は）となりの人に足をふまれた。 I had my foot stepped on by someone (standing) next to me.