You've got to love John Mueller from Google, he posted on Twitter the line "There's no such thing as LSI keywords -- anyone who's telling you otherwise is mistaken, sorry." Here is that tweet:

There's no such thing as LSI keywords -- anyone who's telling you otherwise is mistaken, sorry. — 🍌 John 🍌 (@JohnMu) July 30, 2019

LSI is Latent Semantic Indexing or Latent Semantic Analysis (LSA) - a topic I really have not mentioned here since 2005. It is basically a natural language technique that analyzes the relationships between a set of documents and the terms they contain by producing a set of concepts related to the documents and terms. It is a technique Google can use to understand content and concepts on pages within its index. You can ask Bill Slawski if Google uses it or not.

But we've seen SEOs use that talk before, see this thread where an SEO said "LSI/keywords seems to have become less important and authority more important," when discussing an algorithm change.

It doesn't really make sense to say it that way and here is John reinforcing that.

Forum discussion at Twitter.