To cut to the chase, Flatpak is nice to have, but isn't a complete replacement, and the other suggested "replacement", Pacstall, targets Ubuntu instead of base Debian. If you want more detail, read the spoiler:
spoiler
I use EndeavourOS as my daily driver, so I'm used to the sheer convenience of the AUR. However, I'm not a fan of how that convenience is tied to a continually-changing rolling release distro. This becomes especially apparent whenever a major KDE update comes out, or the Arch Linux people decide to make yet another big change.
Debian is the other main distro I use. I mostly install it on systems I don't intend to use often, but recently I started dual-booting it on my main desktop. I like how slowly things change in the Debian world, and if it were easier to run uncommon software on it, I'd probably use it more.
I know Debian can use Flatpak, and while it is nice to have, it's not a complete replacement for the AUR. The other "replacement" MPR/Makedeb's developer suggested is Pacstall, but that's for Ubuntu and not base Debian. If Ubuntu were the same distro it were back when 20.04 came out, I wouldn't be opposed to using it, but Ubuntu's gone to shit since then.
EDIT: Inb4 Distrobox too.
EDIT2: The reason I want an AUR equivalent is convenience. Yes, I commit the cardinal sin of using an AUR helper when I use EndeavourOS. I like being able to install things and have them integrate into my normal system package management without having to manually set up scripts.
yeah, these things are all trivial to compile.
i suggest not dealing with things like MPR for this; just set --prefix=/opt or something similar when compiling the program, and install everything to /opt.
in slackware this is very easy to deal with, as the package manager is extremely basic and basically just unpacks tarballs and logs its file list somewhere. 'packages' are just tarballs so you just compile anything with a prefix to a temporary dir, tarball that dir and install it with the 'package manager' to keep track of things. this is simple enough so you can do similarly on debian without switching distros or using slackware's scripts