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As far as l know, it is usual to use a comma after a modified noun followed by a non-restrictive relative clause as follows:
The new AIDS treatment, which proved to be highly effective, is extremely expensive.
My question is: Are there special cases that we might ignore using the comma in constructions as the one mentioned above.
To a careful reader, your example, with a pair of commas, is a non-defining relative clause. Consequently, it can be removed and the sentence would both still remain grammatically correct and its meaning wouldn't change.
If a sentence contains an adjective clause introduced by the relativizer which, a careful writer would firstly determine whether the subordinate adjective clause is defining or non-defining and would use commas or not accordingly.
The use of the relativizer that is different. Usually we don't use a comma before it.
No, there are not. That's because omitting the commas would indicate to readers that you are using "which" to introduce a restrictive clause, basically as a substitute for "that." While "that" can't be used in lieu of "which" to create a nonrestrictive clause, "which" can be used in lieu of "that" to create a restrictive clause (see def. 3 - https://www.dictionary.com/browse/which). As such, omitting the commas expresses that "which" is being used restrictively, not non-restrictively. In grammar, it is the commas that set non-restrictive clauses apart, so leaving them out changes the meaning of the sentence.
Does that mean that nobody ever omits the commas while meaning it to be nonrestrictive? No, it doesn't mean that as there are people who write ungrammatically.
Personally, if I had my druthers, I'd just call it ungrammatical to use "which" to mean "that" and do away with the need for commas around such clauses. I believe that would be extremely practical since the only way in spoken English for a listener to discern whether an ensuing relative clause is restrictive or nonrestrictive is whether the speaker has said "that" or "which," respectively, to introduce it, the written commas generally not reflecting any discernably longer pause in actual speech. Were "which" somehow decreed to be used exclusively for nonrestrictive clauses, never restrictive clauses, there would no longer be any reason to require commas around nonrestrictive relative clauses introduced by "which" and we could spare ourselves a whole lot of commas. Alas, I haven't the authority to issue any such decree, though, so it is what it is.
