'persistent' option has no effect

Please try with a newer efi-pack (it works for me in version 18.04 LTS of all Ubuntu family flavours (including Kubuntu). I use usb-pack_efi-2.02.tar.xz which matches "1" in the menu in 18.04 LTS.

If you try to use mkusb with 'too old' versions of Ubuntu, you may have problems. I would recommend that you stay with currently supported versions of Ubuntu. mkusb may or may not work with older versions of Ubuntu (there is no maintenance for end-of-life versions).

If you want to explore end-of-life versions of Ubuntu, I suggest that you try some other tool to create USB boot drives, for example Unetbootin.

I'm trying to create a 'persistent' LiveUSB from LMDE 5 'Elsie' according to these instructions:
https://www.linuxuprising.com/2019/03/create-persistent-storage-live-usb-with.html

mkusb does its job without errors and creates the following partition structure:
https://ibb.co/vvs4z07

There are several new items in the grub menu with the 'persistent' option
in the name and in the settings. However, no system changes (language, time zone, etc.) are saved
when booting any of these items.

@ paul yarkov (snoopy-coder),

mkusb's tools for persistence are developed and maintained for current Ubuntu and Debian versions. Linux Mint 'standard' editions have a boot structure that is close enough to that of Ubuntu, so that it works to create persistent live drives.

First I tested LMDE 5 with mkusb's dus-persistent tool, and it failed like you describe. The boot structure is too different, and I am not ready to fix it, because LMDE is outside the main target for mkusb.

Then I tested LMDE 5 with mkusb-plug and selected persistence according to Debian. It worked. The result is a working persistent live system, and it works both in UEFI mode and BIOS mode (alias CSM alias legacy mode). This method uses 'semi-cloning', actually using sed to replace the boot options 'quiet splash' with 'persistent ' in both cases 12 character strings, and write the output to the target device. There are also some actions in order to create the partition for persistence. See the attached screenshot.

I hope and think that mkusb-plug will work for you too :-)