It's not this forum, it's the default behavior of HTML unless you specifically opt in to significant white space, which few sites do because it has a tendency to create spurious newlines unless the code is maintained very carefully.
I'd be very curious to know if you have any examples of a website that does preserve multiple spaces after periods—I would suspect that in most cases where you think that is happening, it's actually because the font handles the extra space.
Whether it's a bummer or not, the point is this is how every website has worked for the last 30 years.
I find that if you look closely at old books, the spacing is pretty variable. I have one book by my desk from 1895 and the spacing after a period is more like 2½ normal spaces, but I have another book from 1978 and the spacing after period is basically identical to the spacing after a word. There's not much consistency from book to book.
Yeah, it makes sense for HTML -- there is no easy way to differentiate sentence-ending periods from other periods. This is just one of those things that gets worse as technology improves. Another example is phone connections -- phone connections used to be fantastic, and they worked even when the electricity went out, but now we have cell phones, which commonly have terrible connections and are dependent on electricity. Just another example of how advancing technology degrades the user experience in order to achieve other efficiencies.
In general, technology makes things worse but cheaper. :-)
A book printed with letterpress is nicer than a laser printed book, and an illuminated manuscript is even nicer! There are some technology shifts where the new thing is strictly better, like DVD to Blu-Ray, but the majority of the time you have give something up to move forward, like losing the ability to record when we left VHS behind.
AI is going to really accelerate the trend by being really bad at a lot of tasks, but good enough to make do with.
You want non-breaking spaces for this. They're the same width as a normal space, but don't collapse together in HTML.
I once encountered a bizarre bug caused by a user whose keyboard was somehow configured to automatically insert these if they added two or more sequential spaces.
And web browsers muddied the water a lot, since rendering two spaces is generally done by rendering two spaces, unless you're on the web and then have to do something special (some people do! is pretty common). Personally I think this alone is the biggest influence that trained generations that one space is the majority and therefore the most correct.
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I wish they called the zero-width spaces "non-space", so we could have both (non-breaking space) and &zwsp; (breaking non-space).
One. One.
Two. Two.