Recently I installed a forum for testing. I used phpbb on it but later I realised that it has a big demerit that the URL it gives does not contain the content of title of thread. I checked this forum and here also the issue is same.
Its never a demerit for this forum not including title of thread in the URL because it can still be included.
I think you missed OP's point. I think OP refer to either SEO boost or ability to let people guess topic of the link without opening the link itself.
Recently I installed a forum for testing. I used phpbb on it but later I realised that it has a big demerit that the URL it gives does not contain the content of title of thread. I checked this forum and here also the issue is same. Was this not taken into consideration while choosing forum software ? Or does it not matter much? Are there some special benefits of SMF over other popular forum softwares like xenforo or vbulletin?
Changing topic titles may mean URLs also get altered. Some good reasons exist, however, for keeping URLs static rather than dynamic. Firstly, search engines can index unchanging URLs more easily. Second point - user-friendliness; folks bookmarking threads need assurance the URL leads back no matter if tweaks occur to the title.
But it could be prevented if the forum software either create multiple permalink or preserve current URL even when the title is edited.