The suggestion is almost purely cosmetic; Google doesn't care. I guess if you decided to blaze your own trail and make all of your page extensions .jpg then you can deal with the fallout of such a silly decision.
Yes, file extensions can be considered ugly but maybe some people think they're beautiful. Think about when's the last time you had to manually ...
A document that is not often read but should be:
https://www.w3.org/Provider/Style/URI.html "Cool URIs don't change"
To extract the parts relevant to your question:
So what should I do? Designing URIs
What to leave out
Everything! After the creation date, putting any information in the name is asking for trouble one way or another.
You have several options for removing extensions. You don't have to create a directory for each file.
Use Apache's mod_negotiation with multiviews. You enable it with Options +multiviews in your .htaccess file. Then you can link to <a href="some-page"> and Apache will find some-page.html and serve it. Same goes for <img src="...
"(file extensions) can reveal the underlying technology of the website making it very slightly more vulnerable to hackers" is humorous. It is trivial to find out the underlying technology with or without extensions.
Your idea " to put every html file on a website in its own personal subdirectory (giving every webpage the file name index.html ...
It sounds like you have option of just using a single URL. Do you need the path('wiki/main_page', views.main_page) configuration? It sounds like you could just omit that. If you only use the simple wiki/ URL, you don't have to worry about duplicate content or redirects.
On most systems the duplication is caused by the directory URL being powered by a file ...
It's better to use redirects. Multiple URLs with the same content is bad, as described in the link you mentioned.
Actually, you can use the canonical header or meta tag, but it has many cons. And Google says:
Use 301 redirects to tell Googlebot that a redirected URL is a better
version than a given URL. Use this only when deprecating a duplicate
The canonical URL should be the current URL. so your code between <head> codes should be like this in the new URL:
<meta property="og:url" content="https://digimoncard.io/deck/green-otk-1241" />
<link rel="canonical" href="https://digimoncard.io/deck/green-otk-1241" />
If your old URL is you need to ...