Neither is any long or short in Serbian, by itself. However, they become long and short in different words, and not for effect. The word нос (nose) or пост (fast, as in not eating) which have very long o, as opposed to стоj (stop), боj (battle), very short o.
I am certain, however, that at some point there was a phonetic distinction because one can see a trace of it in Bulgarian, and now you told me that in Serbian the distinction is apparent.
The disticntion is apparent in how it evolved. In the western dialects the "ѣ" evoled into "iye/ye" and in the more farther regions (in Dalmacia) into "и" (as in Unkarinian), while in Eastern Serbian, and Bulgarian it evoled into an "э", sometimes even into an "ээ"!
Thus, within the same language, words such as свѣт is pronounced as as свэт, свиjеt and свит, and spelled accordingly, rather than just свѣт.
My impresison is that the Bolshevik orthographic reform replaced the "ѣ" not only with "e" (that is ye, which is okay) but also with "я" in some words. I know that "ѣ" was a YE because the Serbian writer and vladyka from Montengro Petar Petrovich Nyegosh spelled his surname as Hѣгошъ (modern spelling Његош). So, how did it beocme a "я" in some words?
But on the other hand, a 12th century epistle to the people of Dubrovnik by a Bosnian lord was signed by the scribe Radoye as "Ѣ Радое" using the yat in place of Я!
and Я is muted because it is not under stress. E or И would be likewise muted when unstressed, and all three would sound alike
Yes, that's the case wiht modern Greek too. You have I, Y, and H all pronoiced as И. Of course nothing exemplifies this better than English.
I cannot think of an example of that in Russian, but again, I never studied this matter,.
Generally, orthogoraphy was not on people's mind till the spread of literacy in 19c. Getting off the Slav set of topics, Sir Raleigh is known to spell his own name differently within the skope of the same page.