What is that word that means when something is done because it once served a valuable purpose...

What is that word that means when something is done because it once served a valuable purpose, and while it is no longer needed, it continues to be done simply because people expect it? I'm having a mental block here.

Marriage

Democracy

comme d'habitude
force of habit
rite
tribute
homage

Maybe antiquated?

Routine? Ritual? Habit?

You could describe a 'vestigial' tradition, though that doesn't quite hit the mark.

habit

private property

Obsolete?

I don't think there is a word for this, the closest I can think of is 'vestigial' in the biological sense

Traditional might work, ceremonial too?

Vestigial?

Out of habit
(Habitual, habitually)
Routine
Mindless ritual

Give the context. What activity are you talking about?

Shibboleth.

Skeuomorph

Fuck, lit isn't as smart as I thought.

Skeuomorph is design-related. What OP seems to be asking is more of a general word.

why don't you write around it you fucking fag

How long did it take for you to learn German?

Anachronism?

Also what's that word that means an expression of a anachronistic trait, like nails are the this of claws

That sounds like vestigial or atrophied.