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 Department has been automatically applying extension of awards of Personal Independence Payment (PIP) and have issued around 756,000 notifications to date. We still have c850,000 notifications to issue and remaining customers will be notified of their new award end date early 2021, the planned completion date for all extension activity.
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 expect a renewal form whenever it was originally due - although it may be a bit late.