What is the best practice when it comes to old and discontinued product pages? noindex, follow? 301 redirect to a similar product? Put an advisory on the page that it has been discontinued with a link to a current similar product?
I have never done eCom, strictly info sites, however I do know what I like from a searchers perspective.
That last is my preference: advisory on the page that it has been discontinued with a link to a current similar product.
I don't know what your product pages look like or what info is on page or linked to but often a person with an older product or thinking of buying an older product is looking for info about it. Yes, it means that they are not buying. However, it never hurts to be known - by visitors and SES - as a repository of information about the products you carry and carried.
It may open up affiliate parts/repair opportunities. It may, as well, convince the visitor to buy new from you.
I like these kind of pages ... but I'd rather not see them in the first place. Too many webmasters (ecommerce primarily) hang on to outdated inventory pages too long, spoiling the serps and irritating me.
Also, check your logs to see if you can determine where they came from (referrer) get that backlink updated.
You can be more proactive than that. Google Search Console has extensive lists of backlinks to your pages, so you don't have to wait for clicks on old links to show up in your logs to identify what needs updating.