21CN

Is there a time frame when all exchanges are to be upgraded to 21CN?

Is it true that your exchange has to be 21CN before FTTC can be installed?

No guaranteed timeframe. Some exchanges may never see 21CN before they close.

FTTC can be delivered by another exchange in the area, so for example there is no need to installed the fibre kit to the exchange when its just a 100 line exchange, but better to run the fibre a few miles to a larger handover node.

Time frame, not that Im aware
21CN is only required at the handover site to provide FTTC/P. There maybe half a dozen or more exchanges connected to a single handover site.

That sounds like what's happening for me.
A 20CN exchange with no 21CN/WBC eta.

But www.superfast-openreach.co.uk/where-and-when shows an estimate of June for FTTC via BDUK.

You say "before they close". How about those that don't take up FTTC?
Many people are happy with their standard ADSL connection.