I'm using it without any plugins and disabled autocompletion anyway, because it just gets in the way irrespective of the shell. Like not completing tar filenames if it doesn't like the extension (tar axvf) even if the file is there. Modren distributions 'helpfully' enable completion by default.

I don't like the fact that you have no way to clear the history buffer if you've accidentally dropped a password at the prompt. Bash has history -c. I don't have shared history enabled, you could unset that and it'll stop segregating.

Anyway, zsh is a lot more versatile and familiar than tcsh for instance, so it's almost always my go to shell on FreeBSD as /bin/sh is quite limited, but also miles ahead of the Debian shell regarding features.

This is not typically the default setting in zsh (it is if it’s acting like ksh.) Your distributor has probably “helpfully” set it up this way. man zshoptions and look for “SHARE_HISTORY”.

It's Ubuntu 18.04 with Oh My Zsh