The death message death.attack.witherSkull ("%1$s was shot by a %2$s's skull") is only gramatically correct in the obvious case of an unnamed wither killing the player with their skull projectile. However, if the wither is named, or if another entity is able to fire a skull for whatever reason, the structure can become problematic.

For example, it may become:

• "Cultist_O was shot by a Fluffy's skull", which is a problem, because a propper name shouldn't have "a" preceding it, nor a possessive "'s".
• or "Cultist_O was shot by a Evoker's skull", which is a problem because an improper noun starting with a vowel should be preceded by "an" rather than by "a".

Other death messages use a different structure to combat this, generally using the mob type as a propper noun, such as "Cultist_O was killed by Zombie using Master Sword")

Therefore, I would recommend one of the following:

1. "%1$s was shot by a wither skull" (committing to calling the projectile a wither skull regardless of origin)
2. "%1$s was shot by %2$s using their skull" (very in line with other death messages)
3. %1$s was shot by a skull from %2$s"
4. %1$s was shot by a wither skull from %2$s" (a combination of 1 and 3)