Are cancer patients more prone to flu complications even after they complete cancer tx? what about strep throat complications if no antibiotics tx?

Likely yes.: Cancers especially leukemias and their treatments like chemo. Have an effect on the ability of the immune system to fight diseases like flu and strep throat. Therefore it is wise to take antibiotics as strep throat can lead to more serious complications like pneumonia and sepsis where the bacteria get in the blood stream.

Immune problems: Cancer patients might have immune problem, making them more susceptible to infection. Strep throat should be treated with antibiotic to eradicate the strep organism (if it's group a beta-hemolytic strep); if left untreated, it can trigger an immune reaction which causes rheumatic fever (fever, joint pains, heart valve scarring) and/or post-strep kidney damage (post-strep glomerulonephritis).