Hari i want what humans see should be in HTML code, there should not be two versions like no-js mode for bots and a different version for humans like every other software.
This would defeat the purpose of Flarum's extensibility and SPA. It would most definitely require a massive recode of the source code, and would likely make the response size bigger and slower, which would contribute towards negative user experience. It is also not modern to render the entire HTML server-side in today's internet.
IMO right now, SEO just works - we have the SEO extension for robots.txt & sitemap, and some other extensions add SEO tags too (like FoF Best Answer that shows best answers in the search results), which improves it.
Also, spiders and web-crawlers have evolved over the time. Some of them can even execute JavaScript. And those which don't will combine your sitemap with links from the HTML, which will allow them to record a decent amount of content at your forum.
I'd say that SEO is not really just about Flarum, but also about your users. More specifically, the content that they create. Unique and rich content will obviously make your page rank higher for search engines, so keep that in mind. And it is up to forum admins to register their websites to search engines and setup the SEO extension for even better results.
I get the point that you're making. I've seen a couple of discussions about Flarum's SEO support, and it might currently not be at 100 %. But let's not forget that the developers are putting their effort into a vast amount of other things that might have higher priority than SEO at the moment. But this doesn't mean that SEO will not be improved - you can check this issue about search engine optimization for some insights
