Yes, although the server will not ignore the deletion activity if that server is running Lemmy. We’re talking about Lemmy here, not the fediverse as a whole. OP singled out Lemmy in the post title and said “lemmy devs are not concerned with…”
I’m sure there is more to be done in this area. It’d be great to know for sure which software treats deletion activities properly (I’m really unsure about Kbin, I think it does not) and which does not so instance admins can make informed decisions about who they federate with. Perhaps this information could be made available right within the UI that Lemmy admins use to control their instance, rather than an obscure documentation page somewhere…
IMO having deletes federate should be part of a minimum standard all fediverse software has to meet (plus mod tools, spam control, csam filters, etc) before it is allowed to federate but obviously we’re nowhere near having that sort of social organisation.
As long as a deleted post is no longer visible in the publicly-accessible parts of the site, that would be enough verification for me.
I don’t know how the GDPR authorities verify compliance with mainstream proprietary closed source apps, do you?