Notams are restricted to characters used by teleprinters (Baudot Code). No lower case letters and other essential characters used by web URL's.
Lower case or upper case doesn't matter all that much for the web. The DNS portion (server name) of the URLs is case insensitive in any case, and the filename portion can be made case insensitive, depending on the server setting (for instance using Apaches mod_speling or mod_rewrite modules). And furthermore most URLs will just contain slashes, colons, dots and perhaps the occasional dash and question mark as special characters. All of which are available in the Baudot code.
I'm not saying any arbitrary URL can be expressed within the character set of the Baudot code, but that doesn't mean it's impossible to create a valid URL within the limits of the Baudot code.
In the Netherlands, there are several NOTAMs that actually refer to a web page which has more details and a map. The NOTAM about the SRZ Schiphol is one example. Unfortunately such NOTAMs are still the exception rather than the norm.