Git doesn’t “use” namespaces in the sense like they are required. It offers the completely optional ability for a user to use namespaces if they want them. You can use the exact same naming convention as you did in mercurial if you want, and I imagine that’s what the vast majority of people do.

What namespaces buy you is the ability to combine multiple repositories into one, but be able to only show branches from the one virtual repository. Within the namespace, you can still name the branches using whatever method you prefer. Personally, I don’t really see the benefit over just using multiple repositories, but obviously someone thought it might be useful sometimes.

Git has a ton of features, of which maybe 3-5% see daily usage by the typical developer. The trick to getting along with git, is for those features you don’t want/like/understand, don’t worry about them.

Git doesn’t use namespaces for branches.

A legal Git ref (like a branch) is allowed to look like a filepath as
seen on Unix systems like Linux, which means that e.g.:

• feature/xyz is allowed
• feature/zzz/subtask1

Are legal refs.

While these are not:

• /
• feature//zxy

So you may use this permissible syntax to implement informal
“namespaces”. If you want. But it’s not some kind of feature.[1]

All you have to keep in mind that if you create a branch like:

• test/featurex

Then you can never create a branch named test:

$ git co -b test
fatal: cannot lock ref 'refs/heads/test': 'refs/heads/test/featurex' exists; cannot create 'refs/heads/test'

Just like you couldn’t create a directory test and also a regular file
test in the same directory.

Example

The current Git maintainer uses the prefix <initials>/ for all topic
branches in his repo. For example (fetched today, 2023-04-16):

ab/avoid-losing-exit-codes-in-tests
ab/bisect-cleanup
ab/bundle-wo-args
ab/cache-api-cleanup
ab/cache-api-cleanup-users
[snip]
cw/ci-whitespace
cw/doc-pushurl-vs-url
[snip]

Note: don’t confuse this with gitnamespaces

Don’t confuse this “namespace” branching practice with gitnamespaces
which apparently[2] allows a single (remote/server) repository to be
used to serve refs from multiple repositories (because the refs are
stored under refs/namespaces/[…]):

Git supports dividing the refs of a single repository into multiple
namespaces, each of which has its own branches, tags, and HEAD. Git
can expose each namespace as an independent repository to pull from
and push to, while sharing the object store, and exposing all the refs
to operations such as git-gc(1).

Notes

1. And you may as well use --, & or whatever else (legal character)
   as the “namespace separator”. But / does look nicer. 😉
2. man gitnamespaces, Git 2.40.0

Never done something serious with mercurial, so I can miss something.
I’m using namespaces in git branch naming pretty much, but I’m never inserting my name into the name of the branch.

For each task that are describe in task tracker we are creating a separate branch, with number of the ticket and short description of the bug. At this point there are no spaces yet:

  remotes/origin/275-test-new-urls
  remotes/origin/283-fullscreen
  remotes/origin/299-current-menu-item
  remotes/origin/301-users-api
  remotes/origin/306-range-window-resize
  remotes/origin/307-dropdown-init-onload

Then, say, you realize that some task need additional branches for experimenting. These branches are de-facto children to “main” task branch, i.e. after completion the should (if should at all) be merged to that very task. Here come namespaces:

 remotes/origin/283-fullscreen-buggy-animation   
 remotes/origin/283-fullscreen.refactor

I’ve seen that slash is also used for namespacing, but personally I prefer dots, since using slashes can produce some issues.