A parliamentary written answer from Justin Tomlinsion (Minister of State for Works and Pensions) on 11 Jan says that over half of those who have had a PIP extension have not yet had a letter saying so:
https://questions-statements.parliam...0-12-17/131499
Note that an extension to your PIP award end date does not mean that you won't get a renewal form.
The extension is not to defer renewal to a later date, it's to give more time for it to be carried out, you should still get a renewal form whenever it was originally due.