Single user install (was: Wild hair)

>> I've been running SpamAssassin for longer than seems realistic (2.28 
>> with 1998 Red Hat Hurricane?). Anyway I have started moving 
>> everything mail to yet a new machine, Ubunto LTS 24.04.01. That only 
>> supports SA 4.0.0. I've been contemplating the pain of sideloading 
>> 4.0.1 into the system. Then I noticed single user installs. That 
>> seems like a good way to keep a stable 4.0.0 mail server running that 
>> updates when Ubuntu gets around to it and a work 4.0.1 for Loren 
>> writing his rules.
>>
>> Is such an attempt likely to blow up in my aged face or is it likely 
>> to work like it I intend?
>>
What is an Ubuntu "single user install"? Is this like a container or snap?

For other custom services (like WordPress or NodeRED), I'll create a 
user just for that to sandbox it away from the rest of the system. I try 
to do that on my Raspberry Pi projects, but it can be difficult if the 
service authors didn't write for a mortal user. I've yet to deploy 
PiHole because it seems to insist on having root to install. (Presumably 
because it needs a privileged port for DNS, but there are ways around that.)