The ISSN is an international code that's used to identify the title of serial publications (see here the British Library's explanation of the system). It's free to obtain and most poetry magazines have traditionally worn one.
An ISSN isn't related to legal deposit, but it does facilitate the presence of a magazine in libraries, which use it as a fundamental identifier. Moreover, an ISSN enables poets to register their publications in magazines with ALCS. This means that poets might well end up getting money indirectly for their poem even if the journal in question can't afford to pay its contributors directly.
All of the above brings me on to the crux of my post: several excellent new print-based poetry magazines have appeared in the past few years in the U.K. with a worrying trend of not displaying an ISSN. Is there some reason for this that goes over my head?
Poet Ira Lightman's Facebook page has this – perhaps the most bare-faced example of plagiarism that he has yet investigated. This time it's not poetry, but...