+ the subtraction of articles (a, an, the)
+ the subtraction of the second part of a verb contraction (e.g. “‘ve”, “‘ll”, “‘s”, “‘m”)
+ the replacement of a few common homophones with one of their counterparts (e.g. replacing “their” with “there”, “then” with “than”)
Oooh, that's _very_ tricky what they're trying to do there."Perturbations" that cause grammatical sentences to become ungrammatical are _very_ hard to create, for the absolutely practical reason that the only way to know whether a sentence is ungrammatical is to check that a grammar rejects it. And, for English (and generally natural languages) we have no (complete) such grammars. In fact, that's the whole point of language modelling- everyone's trying to "model" (i.e. approximate, i.e. guess at) the structure of English (etc)... because nobody has a complete grammar of it!
Dropping a few bits off sentences may sound like a reasonable alternative (an approximation of an ungrammaticalising perturbation) but, unfortunately, it's really, really not that simple.
For instance, take the removal of articles: consider the sentence: "Give him the flowers". Drop the "the". Now you have "Give him flowers". Which is perfectly correct and entirely plausible, conversational, everyday English.
In fact, dropping words is de rigeur in language modelling, either to generate skip-grams for training, or to clean up a corpus by removing "stop words" (uninformative words like the the and and's) or generally, cruft.
For this reason you'll notice that the NUCLE corpus used in the CoNLL-2014 error correction task mentioned in the OP is not auto-generated, and instead consists of student essays corrected by professors of English.
tl;dr: You can't rely on generating ungrammaticality unless you can generate grammaticallity.