The short answer is no. The long answer: Our immune systems aren't like muscles that need to be worked out to grow strong. By the time we reach young adulthood, we are (under normal circumstances) exposed to so many microbes, pathogens, spores, allergens, etc that we have very robust immune responses, even to things we haven't been exposed to. This isn't necessarily because we have the antibodies for every disease we've been exposed to being produced and floating around our bodies at all times as "strength" would suggest. Rather, our immune system maintains "the plans" for those antibodies so if we are re-exposed to a pathogen or encounter one that's similar to one we've had in the past, our bodies can quickly and efficiently drum up the antibodies necessary to kick it to the curb.
Coming into contact with a pathogen triggers an immune response whether we've been exposed to it previously or not (hence why novel covid-19, for example, didn't have a 100% mortality rate), but this won't make your immune system stronger because that's not how immune systems work. They don't strengthen, they diversify. As a result, low exposure to pathogens in 2020 due to social isolation doesn't weaken or atrophy immune systems. But we don't need constant exposure to different things for our immune systems to be able to run immune responses.
That's not to say that social isolation doesn't have an effect. There's research to show that immune responses are diminished in individuals experiencing stress, loneliness, anxiety, and other negative psychological stimuli. An aggregate of 148 different studies found that people who were more socially connected had a 50% lower mortality rate. One experiment even found that people with many social ties are less susceptible to the common cold which is another coronavirus. This is not a direct effect however, it's indirect.
And almost all bets are off with influenza. As other commenters have mentioned it's going to be very difficult to create an effective flu vaccine for the upcoming season due to the gap in case data from the prior year. Flu almost always jumps from animal populations to human populations on a regular basis so the virus is "novel" very often. Again, we have some ability to mount an immune response to it, but influenza continues to maintain a high mortality rate given how commonplace it is.