Learn English – Are there or were there any other conjoined pronoun-verb combinations like “methinks” in English

Consider if the letter u being represented as v (as I pointed to earlier) — this would be a monumental change which would be unlikely to appear in the modern era, simply because of the number of standards (i.e. ASCII, Unicode, most all information exchange definitions, fonts, most programming language definitions, etc) which would be affected would be enormous.

I don’t think this is a particularly useful example for your hypothesis. If the English language needed to create another letter distinction such as the one that evolved between u and v, many of these pragmatic considerations might be avoidable if we simply borrowed another commonly-available glyph (from another language, a ligature or currency symbol, or whatever) instead of creating an entirely new letter shape. After all, that’s pretty similar to what happened with u and v: as you point out, the two shapes started off as orthographic variants of the same letter. In the 1800s, it would have been extremely expensive to add a new glyph to the metal type on printing presses — so when a distinction arose, they stuck to the glyphs everyone already had on hand.

Spelling reforms might be another “systematic change” in the English language. While it’s true that popular dictionaries have regularized formal spelling in the past couple centuries, that shift began before your hypothesized timing. Also, note that informal spelling has resisted such change; the orthographic shorthand often used in SMS and instant messaging represents one obvious example of the continuing evolution of accepted spelling in informal contexts.

You’ve indicated that adopting new vocabulary isn’t a systematic change, so widespread importation of words when English is forced to co-exist closely with another language doesn’t seem to be a relevant issue either. You also mentioned “structure words”, but those haven’t changed noticeably in the last couple hundred years, either. I’m pretty sure there haven’t been any major changes in the acceptability of different syntax structures in modern English, although I don’t have handy data for that one.

I’m unsure how to get solid data about the frequency of significant morphological and grammatical changes on this 100–200 year timescale, so I can’t address those aspects.

The Online Etymology Dictionary says sustainability is from 1972, though its root words are much older.

sustainable
1610s, "bearable," from sustain + -able. Attested from 1845 in the sense "defensible;" from 1965 with the meaning "capable of being continued at a certain level." Sustainable growth is recorded from 1965. Related: Sustainability (1972).

An article by Nathan Thanki called Sustainable: a philological investigation gives some background, here's an excerpt that neatly links sustainability with Nachhaltigkeit:

So it is what we are trying to sustain that is usually the meat of arguments about “sustainability”—is it overconsumption, overpopulation, environmental degradation? The term has become synonymous with that “meat” in the past few decades. The Club of Rome, in its 1972 report, “Limits to Growth,” claimed that it was searching for a global equilibrium, “a world system that is: 1. sustainable without sudden and uncontrolled collapse; and 2. capable of satisfying the basic material requirements of all of its people.” When the World Commission on Environment and Development (aka the Brundtland Commission) concluded with the notion of “sustainable development,” the emergence of the concept we know too well today was fully underway. Since that time, sustainability has come to be almost synonymous with “sustainable development, defined in Our Common Future as “development that meets the needs of the present without compromising the ability of future generations to meet their own needs.”
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While the origins of the words association with the environmental seem to lie in the emergence of the environmental movement of the 70s, Ulrich Grober points out a deeper root. In “A conceptual history of ‘sustainable development’ (Nachhaltigkeit),” he argues that the term actually comes from 18th Century forestry (at the time timber was a key resource with an uncertain future). German nobleman and forester Hans Carl von Carlowitz wrote “’daß es eine continuirliche beständige und nachhal–tende Nutzung gebe,’ (that there would be a continuous, steady and sustained use).” Sadly, Europe no longer has any primeval forest outside of the Białowieża Forest in Poland and Belarus.So it would appear to me that the quest for “sustainability” is older than we commonly recognise, and, thus, so is our failure to achieve it: marking the failure of civilization.

Edit: The 1972 date is surprisingly late, here are some antecedents from 1906 and 1907.