Privacy is a completely valid reason to change a forum identity, it’s why I’ve always used a persona name on here rather than my real name.
The issue arises when people change their names so many times that if someone blinks and misses just one of those changes, they have no idea who anyone is anymore.
That’s why I whipped up a Discourse plugin* to automagically show the previous name for a fixed amount of time after the change, except when mods overwrite it for privacy reasons.
*Not yet tested. Always a slight chance it may instead try to divide by 0 and crash the universe.