Perhaps it should be written in the guidelines of the list that
posting is an act of giving to the community and this cannot ever be
withdrawn back (and not only due to technical reasons).

Actually, we sort of say that, but not this nicely. The guidelines
say:

Remember, Your Post Will Be Permanently Archived

Because it is a public resource created by and for the entire XSL community, we do not edit XSL-List. Therefore, it is responsibility of list subscribers not to post anything to the list that they do not want publicly available, possibly for a very long time.