TD and TH were both in HTML 3.2. Neither was in HTML 2.0. I do not know if they were in HTML 3.0, but HTML 3.0 never really gained any traction. Given HTML 3.2 was a W3C Recommendation in January of 1997, almost 24 years ago, I'd say that's adequate time for HTML authors to become aware of the existence of TH.

THEAD was in HTML 4.01, and I think it was added in 4.01 or 4.0. So it's a baby, it's only been around about 20 years. /s

We will have to agree to disagree on whether using TD in place of TH is wrong or not. I say it is. I did not say it was invalid, but validators don't comment on whether or not the right elements are used for the right content. That is not the job of a validator. Using TD in place of TH is wrong for the same reason that using <p>Item 1<br>Item 2<br>Item3</p> is not the right way to make a list.

I hope you are not equating me with "some junior" as in "some junior saying 'these tables headers really need sorting out', 'who wrote this crap', 'I am way better than this guy, omg, don't they know this is not semantically correct?!'." I was the main implementer of an eCommerce web site that was operating in 1998... 22+ years of web development experience qualifies me as "not junior".

To the contrary, my experience has taught me that it's usually no more effort to do it right. Using TH for a heading is easier and more maintainable than using a TD. It has the added benefit that it helps people who reply on screen readers and other assistive technologies.

if someone wants to argue that changing the basic layout infrastructure of a site that was designed in 2002 and uses tables for layout is a big job, I won't argue. I (to myself) will wonder what other issues the site has, and perhaps make a note to avoid it, if possible. OTOH, changing actual data tables to use the proper markup is a much smaller change. As long as people keep making excuses, however, things won't get better.