It would be equally as valid to self-host gitea/gogs/sourcehut/gitlab and/or an issue tracker of your choice, which arguably is preferable to adopting a completely different tool over what is a provider issue.
Going against git is an atrocious user interface (if it were good then [1] would be neither funny nor sad). Most people just memorise a few commands and if they stop working they transfer their changes elsewhere, delete the repo, and start again. Sometimes a team will have a “git expert” who has merely memorised a few more commands and is better able to get a repo out of a broken state. Git fails badly at an important for a developer tool: largely getting out of the way.
I've definitely pulled out the BFG here and there to clean up credentials but that's an issue in any VCS.
Maybe I'm biased because I'm "better able to get a repo out of a broken state", but for the record it's definitely not because I've "memorised a few more commands".
I'm by no means a git expert (I've actually just recently learned about bisect for instance), but I have never in my entire career been in a state where I'd just delete the repo and recreate it from scratch.
I've only ever used a handful of commands, the most advanced of which could be probably considered `reflog` when I wanted to revert some changes; or `rebase` (because strictly speaking, it is more complex than merge I guess), but I never ran a command I did not understand or had to memorize.
I actually do share the sentiment about the tool getting out of your way, and my knee-jerk reaction to learning about git internals is just repulsion, because you're right! I'm not there to tinker around with version control, I'm there to solve problems. That said, I've never felt like Git got in my way.
The one and only time I messed up a repo beyond repair was when I deleted some git pack files while trying to delete some binary files from the git history. This is known as user error.
In my day to day use I find that I rarely have to venture beyond rebase, bisect, reflog, cherry-pick, and the standard commands.
In fact I often work with git using the tools that come with it: git on the commandline, gitk for the visualization of the history and 'git gui' for committing work. They might look outdated, but they work really well and are really fast.
There are a lot of modern graphical tools to work with git repos, but some of them introduce their own vocabulary in an attempt to make using git 'simpler', but this just ends up making everything more confusing.
My impression is that a lot of people want powerful tools, but do not want to invest the time to learn how to handle them. The Pro Git book is available to read for free online and after reading the first 3 chapters, you should know about the most important things for day-to-day use. Some people would do anything to avoid reading the documentation: they'd rather spend a whole day checking different guis that make things look easy and familiar instead of spending that time reading the documentation.
Whether self-hosted git or hosting on Github, your issue trackers and such are typically separate from your main repository. Most platforms offer wikis as a side-by-side repository so that should be easy to move, but the rest is at the whims of the platform.
The GP is claiming they moved to fossil because the one repository contains all of this data.
I haven't followed Fossil, so hearing that it includes things like a wiki is news to me.
As a nice bonus Fossil is a single executable/binary file you can drop anywhere and can act as both the CLI for working with the repository and as the web backend with a bunch of ways to access it including CGI, it's own web server or even as a fake script parser (you can upload the linux binary to any shared host that supports custom script parsers -many do- and use a "script" with a shebang that calls the binary with the path to the repository file, thus allowing you to use Fossil with shared hosting services that do not even know about it).