Signatures are sometimes added using templates which are not (and should not be) substituted, instead of the ~~~~ syntax, typically when they are intended to be multilingual: https://commons.wikimedia.org/w/index.php?oldid=250408757&diff=250408884 Archivebot should support these template-based signatures. Currently template-based signatures are ignored by the bot and treated as if there is no signature.
To avoid hardcoding all kinds of signature templates used across different wikis, we could perhaps use rendered HTML. Can Parsoid be used to convert HTML into wikitext to get a "normal"-looking signature text out of a template-based signature? Or perhaps the other way around, centralizing all of the timestamp parsing for HTML instead of wikitext? The former might be easier to implement because most of the existing archivebot can be reused, while the latter approach might be more stable going forward.