Here's a high-level log of grammar changes by version, with the most effort sppent analyzing 3.6 and before. (The release listed is from tags on master, and probably doesn't take into account backports where the commit id is different.)

Release	Commit	Feature	With __future__
v3.11.0	d60457a6	PEP 654 except* groups
v3.11.0	e8e737bc	PEP 646 variadic generics
v3.10.0	145bf269	PEP 622/634 match statements
v3.9.0	0daba822	parenthesized context manager change
v3.8.0	b5e3ea28 + a965db37	PEP 570 positional-only params
v3.8.0	8f59ee01	PEP 572 :=
v3.7.0	-	-	annotations (on in 4.0)
v3.7.0	ac317700	PEP 492 async/await become keywords
v3.6.0	a721abac	PEP 515 underscores in numeric literals
v3.6.0	52c4e7cc	PEP 530 async for comprehensions
v3.6.0	f8cb8a16 (2016)	PEP 526 foo: int annotations
v3.6.0*	235a6f09	PEP 498 f-strings
v3.6.0*	df395991	def f(a,) trailing commas
v3.5.0	-	-	generator_stop (on in 3.7)
v3.5.0	14acf5f4	f(*a if b else c) fix
v3.5.0	de12b79c	f(**a if b else c) fix
v3.5.0	7544508f	PEP 492 async def, for, with
v3.5.0	025e9ebd	PEP 448 {**x} ??
v3.5.0	d51374ed (2014)	PEP 465 a @ b and a @= b matrix multiply
v3.3.0*	1f7ce62b	PEP 380 yield from
v3.3.0*	6ecf77b3	PEP 414 u-prefix strings are back
3.1*	1c50d117 (2010)	refactor argslist, a no-op?
3.1*	4905e80c	Fix ambiguity in unpacking
3.1*	0c31562a	with can have multiple
3.1*	e3944a5e	PEP 401 <> operator	barry_as_FLUFL
v3.0*	2d735bc0	Allow keywords after *args
v3.0*	828f04ac	3-arg raise becomes raise ... from
v3.0*	992d4a3e	PEP 274 {k: v for ... } dict comprehensions
v3.0*	e7ba4956	True, False, None are now keywords
v3.0*	d59da4b4	class decorators
v3.0*	1bc535dc	No tuple unpacking in args
v3.0*	0368b726	extended iterable unpacking
v3.0*	650f0d06	lambda/for ambiguity removal
v3.0*	e66c8c7c	bugfix from ... import (with 3 .)
v3.0*	dde00289 (2007)	... is a token
v3.0*	52cc1d83	Metaclass changeover
v3.0*	81e9502d	nonlocal x
v3.0*	452bf519	print statement removal
v3.0*	b940e113 + 16be03e4	except ... as x
v3.0*	c150536b	PEP 3107 py3 type hints
v3.0*	4f72a786 (2006)	PEP 3102 kwonly args
v3.0*	52318d62	... is a token
v3.0*	7cae87ca	exec statement removal
v3.0*	86e58e23	set literals
v3.0*	cf588f64	backticks removal
v3.0*	b053cd8f	<> operator removal
v3.0*	477c8d5e	from .. for parent relative imports
v3.0*	49fd7fa4 (2006)	change to if-conditions I don't understand
v2.7*	45aecf45	as and with become keywords
v2.6	-	-	print_function (on in 3)
v2.6	-	-	unicode_literals (on in 3)
v2.5	8ae1295c	as and with are optionally keywords
v2.5	f7f438ba	PEP 328 relative/absolute imports	absolute_import (on in 3)
v2.5	c2e20744	with statements	with_statement (on in 2.6)
v2.5	dca3b9c7 + d074beb6	lambda/for ambiguity
v2.5	f599f424	PEP 341 try/except and try/finally unification
v2.5	37c0844b (2005)	require parens on kwarg generators
v2.5	0d6615fd	PEP 342 adds yield expr
v2.5	409d8f2e	class F() ok now
v2.4	1a4ddaec	multi-line import tuple
v2.4	0ccff074	Require newlines between decorators
v2.4	c2a5a636	PEP 318 (decorators)
v2.4	354433a5	PEP 289 (generator expressions)
v2.3	00f1e3f5	PEP 263 (source encoding)
v2.3	2d3b9864	change to backtick that ends with comma
v2.3	84ee323c (2002)	disambiguate power-of-power
v2.2	1c917072 (2001)	ban a list comprehension trailing comma
v2.2	4668b000	// for floordiv	division (on in 3)
v2.2	5ca576ed	yield statement	generators (on in 2.3)
v2.1	-	-	nested_scopes (on in 2.2)
v2.0	434d0828	**= and friends
v2.0	46dfa5f4	[1 if x] is not a list comprehension
v2.0	0360663e (2000)	print >>file, x and trailing comma
v2.0	803d6e54	list comprehensions
v2.0	7690151c	"extended call syntax" f(*args, **kwargs)
v2.0	d295f120	no-args raise
v2.0	03a7466b + 556440d2	assert statement
v2.0	0dfcf753	access statement removal
v2.0	14f44516	slice syntax changes
v2.0	0bfd6c33	add power operator
v2.0	a996b910	3-arg raise, and more
v2.0	4a1da268	import a.b
v2.0	248a50c1	implicit concatenation, try/except/else, default values in args
v2.0	57531fea + 590baa4a + 12d12c5f	lambdas
v2.0	db3165e6	add exec statement
v2.0	b3f7258f + 25831652	access statement addition, something about class header
v2.0	02334d2b	varargs uses *
v2.0	af82141b	looks like try/except/finally becomes try/except and try/finally
v2.0	e785fbcf	allow newline at end of eval
v2.0	610cdc52	varargs uses * or +
v2.0	526e9096	varargs uses +
v2.0	09cea474 + 6cf1273f	evidently = used to work for comparison
v2.0	68fc3497	global statement, "new" class syntax with bases in parens
v2.0	9eb4f535 + a76fb5b6	lots of operators
v2.0	56f78377	semicolons, continue, dict literals, and more
v2.0	4dae2167	remove dir statement?
v2.0	85a5fbbd	initial commit in current location

* These commits are present in old tags according to git, but the released binaries do not appear to recognize them.

Code samples

These are found in the examples/ directory listed by the version they are "first" added. Some are backported. This could be cleaned up, and entries removed from this README if someone is interested and has time.

If you get python 2.3 through 3.8 installed on your PATH, you can check an installed LibCST against the changes that result in syntax errors.

$ python metarun.py
                                          23 24 25 26 27 30 31 32 33 34 35 36 37 38
examples/py24-0027-generator-expr.py       X  o  o  o  o  o  o  o  o  o  o  o  o  o
examples/py26-0025-print-function1.py      O  O  o  O  O  .  .  .  .  .  .  .  .  .
examples/py26-0025-print-function2.py      X  X  .  X  X  o  o  o  o  o  o  o  o  o
examples/py26-0025-print-function3.py      X  X  .  o  o  o  o  o  o  o  o  o  o  o
...