Even if it were a gray area, the serious penalties would probably be enough to make someone want to blur it out.
Would it? I'm certainly not a lawyer or ITAR expert, but I would think that if you walked through a public space where you couldn't positively confirm that everyone present (and everyone who might view it transitively via video recordings, live streams, etc.) was OK to access the materiel on the shirt, that would be considered an export and you'd be in big trouble.
It's not just physical items like munitions, but also things like transfers of information (blueprints, technical data, documentation, etc.) or services being performed, regardless of where (inside or outside the USA) or how (paper, electronic, verbal, etc.) it takes place.
Have a look at [1] § 120.50 (Export) and § 120.63 (definition of Foreign person).
>if someone looks at the T-shirt and transmits it out the country they'd be the exporter, not the person wearing the T-shirt.
I believe the person wearing the shirt would be considered the exporter, as that is the point where the information moves from (I'm assuming for the purposes of conversation) an USA citizen to a foreign person.
But again, I could be wrong. Safest bet is not to print the shirt to begin with :)
[1] https://www.ecfr.gov/current/title-22/chapter-I/subchapter-M...
Here's an example of T-shirt with a machinegun blueprint on it for instance, for sale in USA without any checks as to your immigration status [note US law considers a device that induces automatic fire as a 'machinegun' despite the fact the device itself isn't really a gun]:
https://ctrlpew.com/product/yankee-boogle-tee-gatalog-editio...
On remand to the district court, and on the eve of changes to the federal export regulations, the U.S. State Department offered to settle the case, and on July 27, 2018, Defense Distributed accepted a license to publish its files along with a sum of almost $40,000.[6][7]
It seems maybe they might allow you to export it, but you'd have to get a license first, even if they were required to issue it to you could take years of lawsuits that a youtuber probably will not pursue?
https://en.wikipedia.org/wiki/Defense_Distributed_v._United_...