Farzanah
There are some evidence based things that can weaken the immune system, but there is no evidence that mask wearing is one of them.
Probably another social media myth.
The only way that mask wearing "weakens the immune system"" is that people are less likely to catch colds and other air-borne diseases, which means that the immune system won't have antibodies to that particular virus. However, those antibodies don't protect a person against any other virus, of which there are many, including Covid-19.
As you say, there are there is evidence that some things can weaken the immune response, eg, tobacco, alcohol, poor nutrition, not enough sleep, stress, some medical treatments such as chemo, TNF inhibitors and corticosteroids, but masks aren't included.