How can you permanently mark thinbloods when vampires regenerate all their wounds and always revert to their original appearance, sooner or later?
Is there a designated Camarilla brander who regularly checks up on all the thinbloods to see if they need to be rebranded?
There is no automatic regeneration or regrowth of lost limbs for thin-bloods and something like branding would be treated as an Aggravated injury, which is a lot harder to recover from for both them and normal vampires. There's also no need to constantly re-brand them when there are already dime a dozen
level 1 rituals that can permanently alter someone's appearance, such as:
Brand - makes scars, burns, tattoos and any other changes to the flesh permanent (renamed to Seal the Brand in V5)
Beat your Way to Glory - beat the superficial characteristics out of someone and onto you (this is what V20 Anarch Sorcery is like BTW)
Rebirth of Mortal Vanity - lengthens your hair
Purity of Blood - expels foreign materials like piercings, tattoos, hair dye, etc.
And before anyone tries to come back with the "B-B-BUT ONLY TREMERE CAN DO BLOOD MAGIC!!!" cope, that's false as anyone can learn blood sorcery and about a third of the clans already have access to it. What makes the Tremere unique is that they have a much easier time learning the Paths and rituals belonging to other clans.