I've mentioned it a couple times in this thread before and it's probably buried since this thread is nearing 2k posts, but yeah I periodically go back and delete promos that are months old.
The reason I do it is because
1-New readers who come to my series and binge my series see over 80 updates are gonna ignore promos if they've just seen over 30 updates of promos. This allows the new promos I do for people to be more effective
2- I have promoed a lot of people who's series are now gone/moved to webcomicsapp/stopped being updated. It's too much effort to individually delete the promos of a single inactive comic, and I dont want to keep sending my readers to "dead" comics because again it makes my future promotions uneffective.
3- I promo a lot of the same people over and over, it's not as effective if the new readers keep seeing you over and over.
4- I personally don't find most promos to be effective after the person has had a couple updates anyway. The majority of bump is in the update itself, and then maybe a small bump the second update, then it just stops.
I'm am totally okay with anyone who has promo me in the past removing my promo as well after it has been a couple updates.