I'd assume that is the purpose of the "Mobile Platform" field on the right side of the bug. Maybe the OS field can't be changed in Bugzilla? Andre will have to clarify on that.

Historically speaking (before I joined WMF), Mobile wanted to have a separate field AFAIK. I would prefer to kill the "Mobile Platform" field (bug 41197 comment 5), but probably not worth to spend time on as we move to Phabricator.

Upstream values don't include any recent mobile platforms either, see its field values on https://landfill.bugzilla.org/bugzilla-tip/query.cgi - related upstream ticket: https://bugzilla.mozilla.org/show_bug.cgi?id=964717

Those RHS fields aren't available when filing a bug. But yes, I suppose the correct answer here is really WONTFIX-PHABRICATOR. ;)