You must love John Mueller from Google, he posted the line on Twitter "There are no such things as LSI keywords – everyone else you tell is wrong, sorry." Here is that tweet:
LSI keywords do not exist. Everyone who tells you differently is wrong, sorry.
– 🍌 John 🍌 (@JohnMu) July 30, 2019
LSI is Latent Semantic Indexing or Latent Semantic Analysis (LSA) – a subject that I have really not mentioned here since 2005. It is actually a natural language technique that analyzes the relationships between a set of documents and the terms they contain through a set of concepts related to the documents and conditions. It is a technique that Google can use to understand content and concepts on pages in its index. You can ask Bill Slawski whether Google uses it or not.
But we have seen that SEO's used that conversation earlier, see this thread where an SEO said "LSI / keywords seem to have become less important and authority more important" when discussing an algorithm change.
It is not really logical to say so and here John reinforces that.
Forum discussion on twitter.