The libexiv2-27 software library seems to be capable of manipulating the IPTC alt text and extended desc properties. ‘apt rdepends libexiv2-27’ suggests that the Gimp indeed does not have this functionality, which is bad, but at least other packages like Krita and a number of image viewers might. There are also AI tools claiming they can semi-automate adding descriptions.
SVGs, whose inline display in Deltachat is not really supported due to currently-inadequate standardization, also have some in-file description, and alternately, metadata that tells screenreaders to ignore them (details).
I haven’t tested specific workflows, but it seems there are adequate tools and standards: it would be feasible for stickerpack author to add the accessibility metadata, if DC surfaced it. And while DC can’t currently render SVGs inline, it could render their alttext instead.