You have already posted this thread: https://eu.forums.blizzard.com/en/wow/t/undeleting-character-is-not-working-no-reply-by-blizz-support/
And received multiple detailed answers about (un)deleting toons.
Though maybe it’s not explained well in that thread, I’ll also link the more detailed explanation for measures against overcrowding: Layers and Character Creation Adjustments on Select Realms
Edit: See bluepost below with the restart and undeleting toons on locked realms.