Let's unpack this.
Malthus looked at the population trend during the time that he was alive, and the amount of food humans were able to produce, and concluded that a continuously growing population would eventually meet or exceed its food supply, resulting in either general misery or starvation and catastrophe. Malthus proposed a number of possible solutions to this problem, which included birth control
The catastrophe that Malthus warned against didn't happen for two reasons. One, the population stopped increasing as quickly. This was due to a number of factors, one of which was the use of birth control, as Malthus suggested. The other reason was the Green Revolution greatly increasing how much food we could grow on arable land, which Malthus did not predict.
So Malthus looked at current trends, predicted a possible future problem, suggested means of dealing with the problem, people took steps to address the problem, and the problem was avoided. The people who solved the problem probably didn't do the things they did directly because of what Malthus said, but it's impossible to say with any surety they weren't influenced by his work at all. As such i would say that Malthus and Malthusianism were not discredited, but were merely rendered irrelevant, at least as specifically applied to population and food supply. Predicting a problem and helping bring about a situation that prevents the problem is not a failure of theory, it is a practical success.
The population crisis seems to have been dealt with. (For the moment at least.) But in a more general sense the concerns of Malthus are still relevant. _Any_ process that exhibits continual growth but depends on a finite resource will eventually lead to overuse, resulting in some kind of stagnation or collapse. It is always important to try and identify those kinds of situations well ahead of time. Not for the purpose of promoting a general feeling of doom and gloom but so that means can be found to either limit the growth (ideally in a positive way, not through draconian restrictions) or circumvent the finite resource and thus prevent, or at least delay, the problem.
And note that although it would be silly to put concerns about population growth at the top of your current list of concerns, it _is_ something that needs to be continually dealt with. We need to make sure that the situation where the general population receives good education, has access to birth control, and participates in a relatively robust economy continues to be true in those parts of the world where it is currently the case, and try to help extend those conditions to those areas of the world that do not currently benefit from that situation.