Is it true that we can't mine ETH anymore on September 20th?
After the Merge, yes.
There would still be PoW forks, but no point as they're likely to be sold afterwards
The one which many people believe to replace the Ethereum we know now is the ETHW fork.We have to see how it behaves before we jump into any conclusions but there will be other coins to mine even if this is not a success.
There is Ergo,ETC and RVN to name a few but there are quite many more of such coins.You can use unmineable then to mine your favorite coin and choose Phoenixminer there in the unmineable miner.There are other possibilities but so far we have to wait even after some days after the merge to have a clearer picture of the situation.