This is a subject which kind of branches off in several different directions but a few points to make:
- Hotlinking is not against the rules here but it is not recommended
- WebP files have been embeddable since the latest forum software upgrade as per Connor's post here. As was also stated, this is separate to the file type being supported to actually upload attachments to the board itself. All this means is that if you are linking to an image of that file type that has been uploaded somewhere on the internet; it will be displayed... as long as they haven't prevented hotlinking/bandwidth theft!
Websites can prevent any media to be loaded outside of the domain it is intended for (ie. its own).
They can do this either by applying a blanket block to ANY site outside of its own hosting or they can block specific websites or IP addresses based on what their tracking and analytics is telling them.
Of course, the actual forum user doing the hotlinking is often oblivious to the fact that it might not work for others because they load it through the site it is intended in their browser and they copy and paste the direct link to it. The image will show for them because it is in their browser cache whereas somebody else clicking that same link for the first time may find that it doesn't work when they click it if said originator has either disabled hotlinking altogether or has blocked FreeOnes because multiple users are linking directly to images hosted by them.
In the ID section of this forum, people will often use a hotlinked image for their ID request threads. Other users will reply to it and say it doesn't work to which they will think "what are you talking about? it works for me" which is why this type of thing has to be explained every now and then for anyone who isn't familiar with the concept of hotlinking or what is is or means.
Quite apart from all this though, it is a good idea not to hotlink in general just because if you do, you run the risk of it no longer working later down the line. Even if sites aren't applying blocks, they might move to a different domain or change their URL structure meaning that the links that once worked no longer do. Sure, most links on the internet aren't going to last forever; that's only natural... but if you upload the image to an image host or FreeOnes itself, the chances of it remaining there in the long-term are far greater. Granted, you can't upload WebP format images to FreeOnes, but the benefits of uploading stuff to FreeOnes in general were outlined in another thread recently. Basically, if you upload stuff - it'll stay there unless it is inappropriate content.
Another thing is... supposing you hotlinked to an image and the site in question didn't like it... they could replace the URL of the image which is being linked to with something embarassing like "FreeOnes are a bunch of *****" so that would be what is displayed in posts on the board rather than the original images.
I didn't want this to be quite the essay it turned out to be but hopefully that covers a few bases.