The last metric in this 2017 study, before the pandemic, showed tipping was between 18% and 19% in, "surveys (that) are aimed at diners who patronize full-service midscale and upscale restaurants". It also shows a downward trend in the last few data points. All things considered, including diners who patronize "downscale" full-service restaurants (like diners), and given the many decades-long standard of 15% tips, it seems to me a safe standard to continue to use. Certainly no foreign visitor will ever face vitriol for tipping 15%.
The 15% standard supplanted the previous 10% standard somewhere in the 1970s and lasted to the early/mid-aughts depending on where in the US one lived. I don't agree that ~30 years is "many decades-long". Further, that 15% itself was an uptick from the prior standard demonstrates that we're dealing with a moving target, for better or worse.
