Such a feature could be easily used to get information which sites are hosted on a server as you just have to write a script that tries out web1, web2, etc. so I think if this feature exists, it should be disabed by default. And such a feature would have to ensure that access to websites which are disabled in ispconfig or are over traffic is not possible.
Using such a alias to test websites will not work with most cms / blog systems like e.g. wordpress, so installing wordpress over such a link would result in a broken install and accessing wordpress that has been installed under the real domain name will fail as well. So in my opinion, such a feature does not offer any real benefits as its mostly usable for plain html sites which dont use cms systems and such sites nearly dont exist anymore. Pleask has such a feature as its much older and years ago when websites were just directories full of html files, such a feature made sense.
So maybe instead of intergrating such a feature in ispconfig, it might be better to write a short tutorial on how to configure mod_userdir on a ispconfig server and inform the user about the risks that i mentioned above as thes risks cant be avoided when mod_userdir is enabled.