The only place where this would be an issue is the Twtxt Search Engine – But as the GDPR also points out:
Art. 17
The one place the “it propagated and I can’t recall it” problem is legally acknowledged is Art. 17(2), and it explicitly scales to what’s technically feasible:
“…the controller, taking account of available technology and the cost of implementation, shall take reasonable steps, including technical measures, to inform controllers which are processing the personal data that the data subject has requested the erasure…”
Best-effort, given the technology. A decentralised, append-only, content-addressed feed is the available technology, and its limits are baked into the standard the law applies. Nobody — not the user, not you — is obliged to guarantee every cached copy vanishes.