I have regretted deleting certain characters before the restore feature came into existence. I’ve also regretted deleting a couple characters after that point that weren’t level 50+ (they had been above 50 previously, but then the level squish happened) on the mistaken belief that I would remember to undelete them before they went away.
If your characters are level 50+, though, you don’t need to worry about regretting it. They will be on the restore list, and if you want them back, you can bring back one character per week at any time.
If the characters in question are meaningful to you but are, for whatever reason, below level 50, I recommend leveling them to 50 before you delete them just so you don’t have to chance it.