Preorder, Back In Stock ‑ STOQ

Easy to install even on a customized theme. It seems to work as described and integrated seamlessly with my store theme. The free plan is useful for those of us with smaller/niche stores (thanks!)

Just make sure to go through all the settings and adjust to your liking. Note, specifically, that the app defaults to sending out four (4) reminders when a product is back in stock, which I found too aggressive. It's easy to change, though.