Internal Linking – What is Best Practice Today – so Confusing in 2019

After doing SSEO (Self-taught Search Engine Optimization) for ten years and watching hundreds of videos and reading bunches of articles and forum posts I am Totally Confused in 2019.

Years of being encouraged to link from product pages >> category pages >> other product pages >> home page and now being advised in various media that linking from product pages is not a good idea.
I was under the impression that linking via key word anchor text from a product page to the home page told Google what I wanted them to rank my site for.
Now it seems to be "don’t link that way – let Google do its job, don’t try to do it for them" … I kinda get that but does that mean no internal linking?

I have one eCommerce site, 9 years old, that has survived Panda, Penguin, Hummingbird and every other algo and ranks #1 and #2 and #3 on page #1 for its primary keyword search and in the top 5 for what I call secondary search terms.
That site has 200 plus product pages that average 550 words and about 9 internal links and 4 external links to other sites, on every page ! … (and KW density of about 6% +).

On a different site ranking at #5, I removed all the links (internal and external) and a lot of ‘flowery or fill’ content in an effort to move it up … but no change (obviously other factors are involved).

So needless to say I’m confused. Seems that my supposed ‘over-optimization’ is not doing any harm at all.

Back to the point ….. what is best practice in 2019 for internal linking for eCommerce sites?