Go backReturn to the wall

Light a candle to say thank you

It works whether or not you can still tell them

If they are here, send the link. Thanks arriving years late is still thanks, and it tends to matter more than the person saying it expects.
If they are not, the candle is still worth lighting. Choosing their name and writing the words you never got to say still means something, and it stays somewhere you can come back to.

What do you write?

The specific thing. “Thank you for the Tuesday afternoons” says more than “thank you for everything”, and it is the detail the person will recognise as theirs.
You have room for a name and a few words. That is deliberate — it keeps it to the thing that actually mattered.

How long does it stay there?

Permanently, and there is nothing to renew. The wall holds 1,000,000 candles; when the last one is lit it is sealed, and the intention is to preserve it in the Internet Archive.
Light a candle to say thank you