I have yet to play with one, honestly. It’s “on the list”.
One of the benefits to using a thumb ring is, using a more asiatic style draw, you can biomechanically draw an arrow a few inches farther back. Like a 32-36″ draw length something really long back like that.
Whereas with a Mediterranean draw “to the ear”, I can maybe draw back 30-31″ at the very most, and that might be over exaggerating my t-rex arms. Shorter even for a more hunting or native American style draw length of “to the cheek” or corner of the mouth, which I tend to be better at using as an anchor point (where you try to draw back the arrow to the furthest consistent point)
Better/longer draw length often equals greater power upon release, provided that form is appropriate in all other areas. Definitely goes into why the japanese longbow has such an insanely long draw, but comparatively lighter draw weight for a war bow. And conversely, you have very high weight crossbow with a very short draw length of only a few inches. Different methods to achieve a good momentum to an arrow or bolt.
Back to thumb rings, I’m sure many European archers, especially those in Eastern Europe where shorter composite bows were more common, would have made use of them.
And to play devil’s advocate to my own argument of “right hand side of bow could have been done”, it Is possible that the artists are showing longbow archers using eastern technique, but sans the thumb rings that might have been commonly used.
It is also, to be fair, a biblical/Saint’s death scene and not usually to be taken literally as far as the style of armor and weapons goes. It could be late medieval longbows, perhaps shot with some weird ancient “style” of archery, as a mashup, to depict the martyrdom of St. Sebastian. The art is so damn beautifully detailed though, as I’m inclined to believe that the artist is showing a release technique accurately that may have been used in their contemporary times.