user341: The value of DEFAULT_CHECKPOINTS_ENABLED is true and changing it will allow nodes using the default settings to accept blocks that would be currently rejected during a potential reorg. Is this change considered a hard-fork, or only changes to clearly enforced consensus rules can qualify as such?

user47827: A hard fork is defined as a relaxation of the consensus rules. (In contrast a soft fork is defined as a restriction of the consensus rules.) Changing the consensus rules to ignore checkpoints by default (assuming the reference client Bitcoin Core defines the consensus rules) would be a relaxation of the consensus rules and hence theoretically a hard fork. However, in reality the checkpoints that have been defined in Bitcoin Core are many years deep and the probability of a re-org back to these checkpoints is infinitesimally small (zero to a huge number of decimal places). So practically it wouldn't have any impact.
Checkpoints and assumedvalid was discussed at this London BitDevs Socratic Seminar in November 2021 on AssumeUTXO.

341: <p>The <a href="https://github.com/bitcoin/bitcoin/blob/e4e201dfd9a9dbd8e22cac688dbbde16234cd937/src/validation.h#L67" rel="nofollow noreferrer">value</a> of <code>DEFAULT_CHECKPOINTS_ENABLED</code> is <code>true</code> and changing it will allow nodes <strong>using the default settings</strong> to accept blocks that would be currently rejected during a potential reorg. Is this change considered a hard-fork, or only changes to clearly enforced consensus rules can qualify as such?</p>
 47827: <p>A hard fork is defined as a relaxation of the consensus rules. (In contrast a soft fork is defined as a restriction of the consensus rules.) Changing the consensus rules to ignore checkpoints by default (assuming the reference client Bitcoin Core defines the consensus rules) would be a relaxation of the consensus rules and hence theoretically a hard fork. However, in reality the checkpoints that have been defined in Bitcoin Core are many years deep and the probability of a re-org back to these checkpoints is infinitesimally small (zero to a huge number of decimal places). So practically it wouldn't have any impact.</p>
<p>Checkpoints and assumedvalid was discussed at this London BitDevs <a href="https://btctranscripts.com/london-bitcoin-devs/2021-11-02-socratic-seminar-assumeutxo/#assumedvalid-and-checkpoints" rel="nofollow noreferrer">Socratic Seminar</a> in November 2021 on AssumeUTXO.</p>
