> I noticed that with the latest master branch setting tab-bar-show
> to "1" does not work work for new frames. On those the tabs are shown
> even if tab-bar-show is set to 1.

Thanks for finding a case that is still unhandled.

> I suppose a hook is needed which applies the correct setting
> to the new frame?

Generally, Emacs core packages should avoid adding own code
to hooks, because hooks are intended mostly for users, such as
for example, configuring to enable tab-bar selectively:

  (add-hook 'after-make-frame-functions 'toggle-frame-tab-bar)

Fortunately, frames provide a better way to set their default values
with default-frame-alist, that tab-bar-mode already modifies.
So doing something similar fixes the problem: