I have heard this before that deleting too much posts can have you loss your current rank. But I'm unsure if it's totally true.
That's true.
If your posts are deleted, you may lose your activity and if you lose actvity, you may lose your rank.
I have never seen any person saying, I was a senior member but I'm now full member.
Read this topic.
Deranked to Jr.Member from Member.Did you read
the post made by DdmrDdmr above? He already a shared a link to the list containing users who have lost their initial rank.