You recall wrong. :) The evidence that tipping is inherently unjust is overwhelming: https://freakonomics.com/2008/03/the-racial-tipping-point/ The post about race but the results very obviously apply to other characteristics too. Though I very well understand that most people don't care much for evidence when it contradicts their anecdata.

Cab tipping culture is totally different and I would imagine varies significantly locally.

This is the study I was recalling: https://www.academia.edu/51293325/Restaurant_Tipping_and_Ser...

Though in looking for that I did find this other study that found racial bias: https://onlinelibrary.wiley.com/doi/abs/10.1111/soin.12056

FWIW it comes off as pretty snarky to provide evidence and then pre-emptively insult the reader by assuming they won’t respond to it, or assume that disagreement will be a result of rejecting evidence in general and not having some more specific objection (e.g., cab vs restaurant tipping culture being different)