workspace 10 is part of upstream's default config, but was missing in home-manager.
The initial "sway: add module" PR (02d6040003) went through multiple iterations and had workspace 10 included for a brief moment.
Until the author removed it in a force-push commenting
> Have removed the last change which added bound ${modifer}+0 to workspace number 10 as this messed up workspace numbering in sway.
The reason might have been, that sway used to sort the workspaces in the order they appeared in the config.
Attribute sets in nix are sorted, but not "naturally sorted", meaning `bindsym Mod1+0 workspace number 10` comes before `bindsym Mod1+0 workspace number 1`.
It's unclear if that's what really happened. A workaround would have been to use `lib.lists.naturalSort` in `keybindingsStr`.
But I cannot reproduce this anymore in any way.
I assume this has been fixed many years ago by now.
upstream config: 020a572ed6/config.in (L113-L134)
4c0357ff87