Hi I am being asked to prove which exchange I bought Bitcoin on from several years ago. I have the BTC address on my ledger. Is it possible to trace backwards?
It is possible to trace it backwards. Some blocks chain explorers show exchange names (and other stuff) annotations on addresses and detected wallets. That kind of information is clues, but no, is not a proof.
The documentation you need is a receipt.
I have no knowledge that exchange CSV archives can be considered receipt. Chances are your corresponding revenue service will accept them, chances are not.
I would suggest you:
Get CSV archives from the exchanges you dealt with.
Inform yourself if your corresponding revenue service allows passing a CSV or requires a fully formal receipt.
Try to get formal receipts from the exchanges you dealt with.
In general, (currently) trying not to sell large quantities of Bitcoin without hiring first proper legal advisory.
Never ever do large goods and services payments with Bitcoin. That's recklessly open doors for revenue services.