Do Scout achievements regarding 'stun' mechanics still work following changes to The Sandman?

Yes. Valve says they updated the achievements. This is from the Jungle Inferno patch notes.

Long-range ball impacts no longer remove the victim's ability to fire their weapon (but the victim is still slowed) We've updated achievements that were looking for disarmed players.

People on reddit confirm it is still possible to get the Moon Shot achievement, which used to require a maximum duration stun. While the achievement description has not changed, the condition is now a maximum duration slow.

Any achievement which required a 'stun' should now instead require a 'slow' even though the text description has not changed.