The 2.0.0 release of PyPDF2 includes three core changes:

1. Dropping support for Python 3.5 and older.
2. Introducing type annotations.
3. Interface changes, mostly to have PEP8-compliant names

We introduced a [deprecation process](#930)
that hopefully helps users to avoid unexpected breaking changes.

Breaking Changes(DEP):
- PyPDF2 2.0 requires Python 3.6+. Python 2.7 and 3.5 support were dropped.
- PdfFileReader: The "warndest" parameter was removed
- PdfFileReader and PdfFileMerger no longer have the `overwriteWarnings`
  parameter. The new behavior is `overwriteWarnings=False`.
- merger: OutlinesObject was removed without replacement.
- merger.py ➔ _merger.py: You must import PdfFileMerger from PyPDF2 directly.
- utils:
  * `ConvertFunctionsToVirtualList` was removed
  * `formatWarning` was removed
  * `isInt(obj)`: Use `instance(obj, int)` instead
  * `u_(s)`: Use `s` directly
  * `chr_(c)`: Use `chr(c)` instead
  * `barray(b)`: Use `bytearray(b)` instead
  * `isBytes(b)`: Use `instance(b, type(bytes()))` instead
  * `xrange_fn`: Use `range` instead
  * `string_type`: Use `str` instead
  * `isString(s)`: Use `instance(s, str)` instead
  * `_basestring`: Use `str` instead
  * All Exceptions are now in `PyPDF2.errors`:
    - PageSizeNotDefinedError
    - PdfReadError
    - PdfReadWarning
    - PyPdfError
- `PyPDF2.pdf` (the `pdf` module) no longer exists. The contents were moved with
  the library. You should most likely import directly from `PyPDF2` instead.
  The `RectangleObject` is in `PyPDF2.generic`.
- The `Resources`, `Scripts`, and `Tests` will no longer be part of the distribution
  files on PyPI. This should have little to no impact on most people. The
  `Tests` are renamed to `tests`, the `Resources` are renamed to `resources`.
  Both are still in the git repository. The `Scripts` are now in
  https://github.com/py-pdf/cpdf. `Sample_Code` was moved to the `docs`.

For a full list of deprecated functions, please see the changelog of version
1.28.0.

New Features (ENH):
-  Improve space setting for text extraction (#922)
-  Allow setting the decryption password in PdfReader.__init__ (#920)
-  Add Page.add_transformation (#883)

Bug Fixes (BUG):
-  Fix error adding transformation to page without /Contents (#908)

Robustness (ROB):
-  Cope with invalid length in streams (#861)

Documentation (DOC):
-  Fix style of 1.25 and 1.27 patch notes (#927)
-  Transformation (#907)

Developer Experience (DEV):
-  Create flake8 config file (#916)
-  Use relative imports (#875)

Maintenance (MAINT):
-  Use Python 3.6 language features (#849)
-  Add wrapper function for PendingDeprecationWarnings (#928)
-  Use new PEP8 compliant names (#884)
-  Explicitly represent transformation matrix (#878)
-  Inline PAGE_RANGE_HELP string (#874)
-  Remove unnecessary generics imports (#873)
-  Remove star imports (#865)
-  merger.py ➔ _merger.py (#864)
-  Type annotations for all functions/methods (#854)
-  Add initial type support with mypy (#853)

Testing (TST):
-  Regression test for xmp_metadata converter (#923)
-  Checkout submodule sample-files for benchmark
-  Add text extracting performance benchmark
-  Use new PyPDF2 API in benchmark (#902)
-  Make test suite fail for uncaught warnings (#892)
-  Remove -OO testrun from CI (#901)
-  Improve tests for convert_to_int (#899)

Full Changelog: 1.28.4...2.0.0