Safari 15.0 shows it fine for me on my iMac running 11.6.

My apologies, I should have checked more carefully. I'm certain I have seen this on Big Sur before, but I cannot verify this anymore. However, I don't see this problem anymore on Monterey, so maybe I'm wrong. But I have also verified this on Catalina just now, and there the problem still exists, I don't see a table of contents there.

The table of contents is generated client side with js, are there any errors when it doesn't render?

I see the following messages in the JavaScript console:

[Info] Content blocker prevented frame displaying https://golang.org/ref/spec from loading a resource from https://ssl.google-analytics.com/ga.js (spec, line 6944)
[Error] TypeError: null is not an object (evaluating 'menuButtonEl.addEventListener')
(anonymous function) (godocs.js:18)
Global Code (godocs.js:381)

It targets this line, which certainly appears to exist https://github.com/golang/website/blob/master/_content/site.tmpl#L45 (and works everywhere else). Given that you said it was no longer reproducible on your original machine, could it just be safari's aggressive local caching? (the class name changed a month ago)

I have checked on Catalina again, and emptied caches through the hidden Develop menu in Safari, and now the table of contents indeed show up. I wasn't aware that Safari is so aggressive at caching, I always thought that a reload should do the trick. Thanks a lot for the help, very much appreciated!!!

Closing as this appears to be a browser issue