[...]Question - I understand the entire "apt" command universe is now...or soon will be on version "apt 3". Has that change already taken place?
See "apt policy apt"
apt policy apt
apt:
Installiert: 3.0.0
Installationskandidat: 3.0.0
Versionstabelle:
*** 3.0.0 500
500 https://deb.debian.org/debian unstable/main amd64 Packages
100 /var/lib/dpkg/status
IF the change has taken place is it still recommended to use the "apt --solver 3.0 full-upgrade" command?
No, you can now omit it; it will be used automatically. But you should still check what apt wants to do.
BTW is "apt --solver 3.0 full-upgrade" specific for use in Siduction ONLY . . . or to any Debian fork?
No. It was recommended by us if apt wants to do things that seem suspicious. For example remove a lot of packages.
According to DistroWatch, Siduction is the only rolling release and desktop-oriented distribution based on Debian Sid.
See above, apt (Debian) uses it automatically.