If you have example .eml files saved, you can directly open a core issue, at least Galaxy/iPad signatures should be possible to remove if we have examples.
I don’t know how the message looks like, maybe there is a way to recognize it by some tags in HTML or something like that. Need to look at actual email source to see. Worst case we can strip known spam signatures if it is really just the text added.
I noticed DeltaChat does not feature EML export for single messages, nor MBOX export for collection of messages. This would have been handy for this kind of work, but I had to install a separate email client altogether instead. Shall I open a separate feature request?
These are varied enough that a contact-specific function to strip out any selected literal string (or regex, for ones incorporating dates etc.) seems like the only way to do it. Not very efficient or elegant, though.