It depends. I've deleted some with a maxed out mount, that's a bit of a bummer, otherwise it doesn't bother me at all. I usually take a screenshot, empty inventory, and gone.
Fan of playing magblade since 2015. (PC NA) Might be joking in comments.
-->(((Cyrodiil)))<--
I deleted a character that was basically only a mule and never had any feel for. Only level 6 or so too. Easy peasy. Had another level 24 i didn't play anymore but was my first character in ESO. Thought deleting would feel weird, but didn't miss him at all once he went poof. Turned out he didn't have a "personality" for me after all.
Then came the last alt (argonian warden) I don't really play, level 20 now (to grab the level 20 reward). But she feels just right, in a rp kinda way even though I haven't rp-ed in this game with other players. I bought and decorated a house specially for her. In fact was my first bought home that was not an inn room. So thought after deleting the other pointless alts (I like clean rosters), what the heck... clean up ... so I just have my main and one played alt left... I just could not get myself to type delete. So now she lives happily in her little humble mud house, still not being played, wallowing in her utterly uselessness. Heck with the storage chests I don't even need her as a mule. But I actually spend a fair amount on the outfit station to dress her up nicely so she looks cool on the character select screen ^^
Deleted my level 50 Templar and Sorcs as I didn't like either class and never played them.
Only regret I have is not doing the quests that gave out recipes as my all my copies of Orzorga's Red Frothgar Recipes that other characters had learned have all completely vanished and are no longer able to craft it for whatever odd reason (I searched every single recipe they can make and it's just gone. No response from ZOS either)
I haven't deleted one since my completely useless Warden. I have all my Mains that I'm satisfied with and I have no desire to create and level up others. And deleting that Warden was a bit hard cause I had her at Lvl50 with crafted gear and everything, so it was a big loss even though ZOS for some reason doesn't want you to DPS with a Frost Staff so my dreams of an Ice Mage were ruined.