Removing Suffixes - .html

I have just had both barrels from an SEO because the site I made for the clients still has .html after the page URLs when "every other site" (his words) does not.

I think he is referring to something like https://alexcican.com/post/how-to-re...with-htaccess/

Has anyone any experience of doing this, does it work with SellerDeck or will it cause havoc?

Thank you so much.