This is untrue. Virtually all plants have all essential amino acids. The only difference is that the main plant-based sources of proteins that people rely on - legumes and grains - have relatively lower levels of one or another amino acid than some other sources. But no studies have found those differences to have any negative impact on people’s health, or even their ability to gain muscle, whether they combine proteins or not.

Here’s an interview with Christopher Gardner - a specialist who goes into detail on this topic.

What we really need is for people to worry more about where they’re getting their fiber, than protein.