I find that the fact that the URL only contains the post number and not both the topic number and the post number is fine.
However, I just observed a side effect which is less palatable.
I had sent a pm to somebody about a topic, with remarks about its contents and with the proposal to delete his two messages and my two messages, at the end, because they were not useful anymore after some modification of former posts. In my message box, the URL stored was of the form
https://www.insecte.org/forum/viewtopic.php?p=1412530#p1412530
. And, you guess it, if I click on this link after the deletion, I get a message about a topic which doesn't exist.I don't know how it would be possible to keep the link between deleted posts and their topics in some place otherwise than in the URL.