Took from google some of the tables, some have it in brackets, some list it, some of them straight up remove it, why is that?
why is the 未然形 for 形容詞 in classical japanese not written/in brackets in most conjugation tables?
Arguably, there is no 未然形【みぜんけい】 for adjectives.
The basic meaning of 未然形【みぜんけい】 is the "not-yet so form". In linquistics terms, this is often rendered as the "irrealis" aspect, used to express something that isn't real -- it hasn't happened yet, or it hasn't fully come into being yet.
Some grammatical analyses treat the form that attaches to the negative ～ない ending as the 未然形【みぜんけい】 by default. This is probably why some sources do list a 未然形【みぜんけい】 for adjectives.
But since adjectives by their very function express a quality of state, rather than an action or change of state, the basic concept of "happening" doesn't apply here. And, indeed, there is no functional irrealis form for adjectives.
Looking more fully at how adjectives (specifically "-i adjectives" or 形容詞) are negated, we take the basic form ending in ～い, replace the ～い with the adverbial ending ～く, and add the negative ～ない on the end. However, we can also stick a particle in between, such as in 高【た】くはない, or 暑【あつ】くも寒【さむ】くもない. This is further syntactic evidence that the ～く ending isn't a 未然形【みぜんけい】, since by definition that must be immediately followed by a 助動詞【じょどうし】 (auxiliary or verb suffix). We cannot insert particles before the ～ない for verbs -- we cannot grammatically say, for instance, *書【か】かもない.
The sources that put this in brackets are probably trying to split the difference, acknowledging that some folks view the negative stem as a kind of default 未然形【みぜんけい】, while also acknowledging that this works differently for adjectives.