Regarding steam and all, you shouldn't install apps in the live session, it will eat your system memory (since the FS is read-only on the medium). The reason they're not available is simply because the APT cache isn't built yet. You can workaround this by refreshing the cache (apt update).
There's no reason for the screen locker to work in live mode, if anything it would get in the way of your installation.
New ISO coming up, with fixes for both issues.
CTRL+Alt+L doesn't lock the screen
When I changed it from modern to traditional it changed my theme to mint x and my desktop to Linuxmint
Nevermind I found it in the welcome screen
couldn't find the setting to set the panel from modern to traditional
couldn't install Steam or multimedia codes from mintinstall. on live usb, It showed as not availavle.
Home dir encryption doesn't work. In syslog, we get an error saying the kernel doesn't support filename encryption, and this results in the user account not being set up properly and we're unable to log in graphically (the home dir doesn't get un-encrypted). This ISO ships wih the same kernel as 19. Ecryptfs is the same as before. The installer ships with a fix to /etc/crypttab, but reverting it doesn't help.
The text plymouth theme doesn't look properly centered.