(I'm going to write this comment from memory rather than look up all the references I'd need to double-check. This means that it's my memory of technical stuff that happened over the past 20 years. My memory of technical details from 20 years ago isn't perfect, so I'll probably get a few things wrong. I'm also writing it using "we" to refer to groups I was part of at the time -- which in some cases are and in some cases are not groups that I'm part of today.)
The behavior that's being removed here isn't really a "bug". Back in CSS1 and/or CSS2, the spec for floating ::first-letter (or, in CSS1, :first-letter), was much more vague. I think it roughly allowed implementations to do standard inline layout, but said that they had the option of trying to do layout better. Gecko (the engine used in Firefox) was the only browser implementation that took that option.
In particular, Gecko's behavior was to actually use the bounds of the glyph (rather than the font metrics for the whole font) to do layout for a floating first-letter, so that there wouldn't be extra space around it and it would align better. This was a better default behavior, but it was also somewhat less controllable since some of the standard inline layout properties (like line-height) didn't apply.
It also turned out that this better behavior wasn't good enough to really do good typographic first-letter effects. Maybe about a decade after Gecko implemented the glyph-wrapping behavior for floating first-letter, some folks (primarily Dave Cramer) who were interested in doing better initial letters came to the CSS WG and developed (over a period of years, with quite a bit of interaction and discussion in the working group) a new set of CSS properties with a substantial spec (at https://w3c.github.io/csswg-drafts/css-inline/#initial-letter-styling ) to address first-letter typography.
At some point during the progres of that work, one question that came up was whether the spec should continue to have this vague allowance that implementations could try to do something better (as Gecko, and no other browsers, were doing). Given that we knew at this point that the Gecko behavior, while better, wasn't sufficient to do good typography, this seemed like the right thing to do. As one of the Gecko representatives on the CSS WG, I absolutely could have objected on the basis that we *were* doing something better and would like to continue to do so, and such an objection probably would have led to the WG not removing that allowance from the spec. But removing the allowance, and moving towards better interoperability, was the right thing to do, so I supported removing it. (That's also when I commented on and reopened the bug being discussed here.)
That said, it also didn't seem like removing the better behavior from Gecko was the right thing to do until we had implemented the *even better* new spec with the initial-letter-* properties, which would allow Gecko users to see better-quality typographic first-letters in the new way. (Though there's an obvious trade-off there between quality and interoperability. The opinions of standards bodies and implementers for the Web platform have changed a good bit over the past 20 years on how to make such tradeoffs -- generally towards stricter interoperability at the expense of allowing implementations to do "better" things.) So, back when I was working on Gecko, I thought that we should keep it until we'd implemented the new initial-letter-* properties. It seems like the folks currently working on Gecko made the opposite call. But I think both decisions are reasonable -- there's a real tradeoff there (though the inputs into that tradeoff are likely changing over time as well).
So, really, just saying "hey, they fixed a really old bug" isn't that useful a point to make. There's much more history there. (Also, see https://dbaron.org/log/20080515-age-of-bugs which I wrote 14.5 years ago in response to general criticism about the age of bug reports.)
