  Blockchain's evolution during the past decade is astonishing: from bitcoin to
over 2.000 altcoins, and from decentralised electronic payments to transactions
programmable by smart contracts and complex tokens governed by decentralised
organisations. While the new generation of blockchain applications is still
evolving, blockchain's technical characteristics are also advancing. Yet,
immutability, a hitherto indisputable property according to which blockchain
data cannot be edited nor deleted, remains the cornerstone of blockchain's
security. Nevertheless, blockchain's immutability is being called into question
lately in the light of the new erasing requirements imposed by the GDPR's
``\textit{Right to be Forgotten (RtbF)}'' provision. As the RtbF obliges
blockchain data to be editable in order restricted content redactions,
modifications or deletions to be applied when requested, blockchains compliance
with the regulation is indeed challenging, if not impracticable. Towards
resolving this contradiction, various methods and techniques for mutable
blockchains have been proposed in an effort to satisfy regulatory erasing
requirements while preserving blockchains' security. To this end, this work
aims to provide a comprehensive review on the state-of-the-art research
approaches, technical workarounds and advanced cryptographic techniques that
have been put forward to resolve this conflict and to discuss their potentials,
constraints and limitations when applied in the wild to either permissioned or
permissionless blockchains.
