Quite a few conventional loans these days are assumable with the lender’s approval. The lender will only approve if the individual assuming the loan can qualify for a new loan under the lender’s underwriting guidelines.
Assumables are still in the marketplace. They are called qualifying assumables.
Gone are the loans that were assumable without qualification.
Tien, I believe you are confusing the due on sale clause with loan assumption. A loan assumption without the lender’s approval can trigger the due on sale clause, but that does not prohibit assumption with the lender’s approval.
I would suggest you take the realtors contracts class if your area offers one. From your questions however, sounds like you do not understand how investors buy with no money down. Most of us do not buy using banks.