I'll admit I never noticed this before - I just noticed this exemption in an old admin post:
https://www.whatifsports.com/forums/Posts.aspx?topicID=109203&threadID=5841893#l_5841893

"Improved the Rule 5 Draft player availability process. Any player acquired the 24-hour period prior to the Rule 5 Draft Roster Freeze is no longer exposed in the Rule 5 Draft. This is to handle the situations where a free-agent signed late, a trade was processed at the last minute or a waiver claim was processed and the owner didn't have time to add the player to the 40-man roster."

Can anyone confirm if this is true? I've got my 40 squared away but I am considering signing a last minute F/A during the last period before roster freeze. If I'm interpreting this correctly, it will basically put this person on an exempt list (technically permitting me to protect 41 players).

Can't believe I didn't notice this before...

This is true.

That's excellent, thanks for the confirmation. I mean... someone will still have to get waived/traded at some point... but still a nice feature.

Look at it this way, protects you from someone else from stealing a player dirt cheap who you would obviously protect on your 40.