Missing feat, mage slayer

Pulled up my 9th level character today and was surprised to see that he had an ability score improvement available.  No way I would have missed that for this long.  So I check, and sure enough, his Feat taken at 8th level is gone.  Mage Slayer is no longer in the pulldown menu for Ability Score Improvement/Feats.

Do you own the Player's Handbook here?

If not, my guess is that you were in a campaign with content sharing on, and for some reason content sharing is no longer on.  That would explain why you can't see  Mage Slayer in the pulldown menu for feats. It does not explain, however, why it went missing from your sheet in the first place, as even when content sharing ends/a character is removed from a content sharing campaign, options already chosen for the character are supposed to stick around. 

I have the 2014 Player handbook but do I have to buy the 2024 player handbook to have this feat?

I have had issues with my last feat going away when copying a character to experiment with builds sometimes.