I understand that this question might come off as a borderline carbon copy of
and feel free to delete if it doesn't bring any new value, but given that quite a while has passed + there were people describing experiences with their systems crashing after trying to fully copy /home directory, I just wanted to re-ignite the topic for a second.
I have an old Dell with Ubuntu 18.04 installed on it clean (not dual boot), and I just got a Lenovo where I installed Ubuntu 20.04 clean as well. I wonder if there's a set of folders that are believed to be really safe to simply copy into the new system? Besides the obviously harmless stuff like basic text files (with notes), spreadsheets, pictures, videos etc etc
E.g. could I copy the bin folder? snap folder? 3rd party installations (e.g. in my case folders containing R/Python programming software)? What about "hidden" folders (like "./config")?
It's just that I do have a fear of trying to use black-box cloning options, while I also don't mind spending time re-building the system from ~0ish. The reason why is because it's not like I've exactly built an empire on the old laptop due to the following: last Fall I tried to update from 16.04 up to 18.04, and it crashed the system without the ability to recover (yeah, I didn't back it up, the lesson was learnt), so I haven't really had a chance to over-customize it too much since then. Hence, you might also understand my PTSD about this occurrence, triggering unwillingness to upgrade my old system to 20.04 for the sake of better compatibility (albeit I'm backing it up).
Your question explicitly states: ABSOLUTELY safe.
Absolutely safe to copy, obviously, is your user data: documents, pictures, etc.
The other items in your home folder are user configuration data. Simply put, copying these is NOT absolutely safe, unless the application that uses them is at the same version as on your 18.04 install. The possibility exists that a newer version changed the format of its configuration files. The presence of the old configuration may in a few cases cause unexpected issues, although that will not be common. But the possibility is not excluded.
In practice, you will likely have no issues in 99% of the cases copying over your old configuration files to the new Ubuntu version. But it is NOT ABSOLUTELY safe.