No worries - I believe you
I was thinking about two ways to detect this:
1. A user setting where the user says "I want at least XXp quality".
This would render some files un-downloadable as long as it is enable but as long as you know that the content is e.g. available in a higher quality than 240p, this would work just fine.
2. This is the one that I will implement for now and also what you've suggested:
If the filesize on download is smaller than during the previous linkcheck -> Display error & wait.
Please keep in mind that this will fail if you add new items while they've already blocked you because JD cannot know (well I assume that it can't) the "expected" quality before.
You should also inspect their website whenever you're blocked:
Can you choose higher qualities but still only see 240p or is it a complete "shadow ban" e.g. once blocked, all other options disappear and you can only see 240p?
-psp-
EDIT
Additionally, please register for a free xvideos.com account and check if their "official downloads" are also affected by this limit.
You can add your free account to JD and it will be used for downloading
EDIT2
Okay I won't change anything for now.
Please check if your issue keeps happening even if you enable the following setting:
Settings -> Plugins -> xvideos.com -> "Prefer HLS Download"