T230897: Measure the impact of native image lazy loading on desktop added the "experimental" NativeImageLazyLoading configuration variable, and support for this in ThumbnailImage.php. This is used in [[Special:NewFiles]] (see https://en.wikipedia.org/wiki/Special:NewFiles ) but not otherwise; git log -S in the mediawiki-config repository doesn't turn up any indication that this was ever enabled in WMF production for any wiki, and the comments on T230897 seem to indicate that there were problems with users wanting to print an article w/ lazy images and the images not being present in the printout.
The loading attribute is implemented as part of the MediaTransformOutput options, but it doesn't look like Linker::getImageLinkMTOParams ever translates a wikitext file option string to this particular MTO option value. So this isn't exposed to wikitext at this time.
This is a placeholder bug for Parsoid support of the loading=lazy attribute on images. It is presumably extremely low priority as long as the NativeImageLazyLoading configuration isn't turned on in production. But if that experimental feature is ever picked up again, Parsoid will probably have to keep pace.
(Note there's an orthogonal issue with the use of loading=lazy for VE CE nodes (ve.ce.MWImageNode) to improve mobile performance, which Wikia is interested in exploring. That doesn't require any support from Parsoid, at least not at this time.)