Not sure if you guys have since figured this out, but it took a LOT of messing around for me to finally fix it. It's "easy" once you do it, but was so roundabout and unnecessary. My iPhone is iOS 18.3.2 currently.
- Open the emoji keyboard in Messages
- Click the Stickers button in the "+" menu (the emojis should become very large in size)
- Click the Clock tab that appears at the top of that menu (on the far left)
- Press/hold the Sticker for about 1-ish second (it will "expand" a tiny bit, showing that you've selected it, and a light glow encompasses it for a second)
- When you let go, a there should be a banner above it that says "Remove." (You can remove Stickers, emojis, and memoji stickers by following those steps).
I have verified that YES it will remove it from your recently used emojis WITHOUT you having to delete the sticker OR the memoji.
I wish I would've tinkered with it for just a few minutes more, because I ended up deleting the stickers. But it was only 2 of them, so it won't be a hardship to get them back.
Happy deleting, y'all :D