I remember a grammatical rule when I learned English that some verbs such suggest, assume, suppose, and so on, require verbs after them be in subjunctive form. For examples,
We suggest that she be released immediately.
Suppose n be a number.
Assume a fact be true.
But I have seen in practice that this rule is ignored (completely) and people simply write as
We suggest that she is released immediately.
Suppose n is a number.
Assume a fact is true.
I wonder if I am wrong and this rule does not exist.
Edit: This question is different from the possible duplicate of another one in that it concerns about a rule in English that could be obsolete nowadays, rather than when to use the subjunctive mood.