What do you think about just setting the height as default?
Wasn't that basically what I ended up suggesting when we last spoke about this? 
It makes me think that maybe a sane default choice for TalkImg would be BBCode that includes just the height attribute...
And then later:
Ha! Reading my own thoughts, I see that I've left you with three options:
(1) Do nothing, and rely on this problem being fixed in another way.
(2) Make the default TalkImg BBCode include both a width and a height attribute.
(3) Make the default TalkImg BBCode include just the height attribute.
I think if it were up to me, the choice would be between 1 and 3. (But, remember, I only have a hunch that for typical forum content vertical layout shifts can mostly be avoided if the browser knows just the height of an image before loading it. So, you should probably do nothing until someone tests that hypothesis. If it turns out that browsers really are super picky about needing both dimensions before they can do anything clever around avoiding layout shifts, then the choice is between 1 and 2, and in that case, the tradeoffs aren't so clear, and with unclear tradeoffs, I tend toward inaction.)
(1) Do nothing, and rely on this problem being fixed in another way.
(2) Make the default TalkImg BBCode include both a width and a height attribute.
(3) Make the default TalkImg BBCode include just the height attribute.
I think if it were up to me, the choice would be between 1 and 3. (But, remember, I only have a hunch that for typical forum content vertical layout shifts can mostly be avoided if the browser knows just the height of an image before loading it. So, you should probably do nothing until someone tests that hypothesis. If it turns out that browsers really are super picky about needing both dimensions before they can do anything clever around avoiding layout shifts, then the choice is between 1 and 2, and in that case, the tradeoffs aren't so clear, and with unclear tradeoffs, I tend toward inaction.)
I remember wondering at the time: "Why did he go for the option that I least recommended?"
Now that you mention it, I remember that conversation. And I also remember having the idea that I had followed the suggestion.
I went to check, and I had indeed applied the suggestion, but only in the test model and not in the final model.
I have already applied the suggestion, as I had thought. Now the embed code by default only includes the height. Hopefully it minimizes the likelihood of disproportionality.