This dell community forum thread says it all
https://www.dell.com/community/XPS/Ubuntu-deep-sleep-missing-for-xps-9310/td-p/7734008
The S3 is disabled deliberately on firmware level.
$ cat /sys/power/mem_sleep
[s2idle]
is missing the "deep" option and thus the GRUB directive
mem_sleep_default=deep
won't have any effect.
So until DELL releases some sane BIOS update which lets users decide on what suspend state they want to use S0 <> S3, I set up the hibernate function on 20.04 this way
https://www.dell.com/community/XPS/Ubuntu-deep-sleep-missing-for-xps-9310/td-p/7734008/page/3
and enabling the status power option "hibernate" like this
Hibernate button on ubuntu 20.04
with additional edits from the comments below:
Following the workaround from the FAQ.. copy paste the below content in the file /etc/polkit-1/localauthority/10-vendor.d/com.ubuntu.desktop.pkla
[Enable hibernate in upower]
Identity=unix-user:*
Action=org.freedesktop.upower.hibernate
ResultActive=yes
[Enable hibernate in logind]
Identity=unix-user:*
Action=org.freedesktop.login1.hibernate;org.freedesktop.login1.handle-hibernate-key;org.freedesktop.login1;org.freedesktop.login1.hibernate-multiple-sessions;org.freedesktop.login1.hibernate-ignore-inhibit
ResultActive=yes
Reboot & Test
Hibernate works. My DELL Vostro won't melt even in a bag. The slight longer boot time is still fast enough thanks to SSD