@Panda www these days is typically an alias
record in DNS, and isn’t a requirement for the site to function. Most websites (including this one ) drop the www part as it’s less for people to type (but it will accept those requests, then strip it back to the actual URL). Only those who have a legacy reason to include www
in their website addresses these days do so.
It’s considered unnecessary in today’s internet landscape. However, if the site presents an SSL certificate mismatch or warning, then this will actually not only harm your SEO (Google penalises against this), but it also means that if you proceed, every transaction on the site will be in plain text - not good at all.
Furthermore, those sites (like NodeBB) that require a specific URL to operate against won’t work properly because of the mismatch, and that site should then be forcing all non-https traffic to https to ensure that this does not happen.
Essentially, not doing so isn’t just bad etiquette, it’s bad for overall security. Depending on the host in use (either Apache or NGINX), there are a variety of ways to accomplish this. One of the easiest ways if you use Cloudflare is to do it with a page rule - failing that, a few simple lines of config is all it takes to resolve it.
A bit more on the whole www part here
https://dropwww.com/why