I had a strange request, a user was asking to add the Notes and Activities onto an entity.
I never knew you could remove the notes and activities from an entity, so I went over to the entity to find the notes and activities cupboard was indeed bare.
I wondered if someone had done something dodgy to remove the fields but no, I googled it but nothing.
I then went to the Entity in solution and found Notes and Activities are tick boxes, which I am guessing are ticked by default when you create a new entity.
So it seems the person who created this custom entity unticked all options.
One word of warning the Notes and Activities check-box’s have an ominous tombstone next to them, this means if you tick this and save it you can never untick it (but you could hide the values I guess)
Hosk's Dynamic Blog 
Whoever created this did the right thing – unless and until you are sure you need Notes and/or Activities, don’t tick the boxes. Once you have a business need for them they are easy enough to tick, but as you point out, you can’t ever untick them again.
Even removing their appearance from the forms still leaves things like ribbon buttons to clean up as well.
And if you have ticked “Activities” then the entity will always appear in the list of choices for “Set Regarding” for any activity (unless the user does not have Append To rights on your custom entity).
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