Plug-ins provide functionality missing from standard Calibre. There is no need for LC to add functionality that Calibre already has for "Identifiers". ISSN is an "Identifier".
If you hover your mouse over the Edit Metadata dialog field for Identifiers, a ToolTip will appear, showing you the format to use. Example:
isbn:9781605207971, lc_authority_name:nr2003021008, lccn:lccn-nr2003021008, loc_lccn:04003431, oclc:779937032, oclc-owi:924827, viaf_author_id:155240756
This may be useful to you:
https://manual.calibre-ebook.com/
DaltonST