Although Xenialpup was published 11/4/2017, it is binary compatible with Ubuntu 16.04 LTS AKA Ubuntu Xenial Xerus . Binary compatible means it was woofed using Ubuntu 16.04 LTS' binaries, and to provide download access via Puppy Package Manager to Ubuntu 16.04 LTS' repositories. Ubuntu 16.04 LTS was released on 4/21/2016. https://wiki.ubuntu.com/Releases. Subsequent releases --e.g. 16.04.1-- were essentially 'bug fixes and/or security updates' not fundamentally different from the original publication. End of 'Standard Support' took place in April 2021. [I think the End of Life Date of 2026 is a notice of when Ubuntu will cease providing any security updates for 'Enterprise-level>used as servers by Web-hosts?].
At any rate, Ubuntu Xenial repos are no longer maintained as active repos, search-able via https://packages.ubuntu.com/. Plugging the name of an application --e.g. gimp-- and selecting an OS version -- e.g. impish-- there will lead to a web-page from which the binary can be downloaded and where its dependencies are listed and linked to, e.g. https://packages.ubuntu.com/impish/gimp. AFAIK, Puppy Package Manager may make use of that information or perhaps that found in a binary's control file.
The binaries of Xenial Xerus and their dependencies are now archived here, http://old-releases.ubuntu.com/ubuntu/pool/. AFAIK, there is no Search Engine, no list of what files are needed by any other files. If you know what you need, you can find it by burrowing into the sub-folders where in a sub-folder, for example, you'll find all versions of libao-common_1.0.0-4_amd64.deb ever published http://old-releases.ubuntu.com/ubuntu/p ... iba/libao/.
Good luck trying to flesh-out your operating system with applications you need to preform the tasks you want.
Alternative to woofing a new Xenialpup: Swap Xenial's kernel (5 minutes). Use its builtin Applications. (0 minutes). Use the new Web-browers available on the Forum. (Estimated download and set-up time, 10 minutes). Use other applications built for Xenialpup or 'No-Arch' available on the Forum (estimated time, same as preceding) and some AppImages which will function under Xenial. (Can't estimate time: some --e.g. LibreOffice, Gimp-- will work OOTB; others may require testing multiple versions until an Old version is found which functions, or you conclude none will function or you give up). When and if Xenial is no longer able to use current web-browsers or access web-sites, create a chrooted Web-browser running a current Puppy as the chroot. (Estimated time 2 hours).
Alternative 1.5: Swap the kernel. Update Xenial's utilities, e.g. Radky's JWMDesk, rerwin's networking applications. Change the Wallpaper and add some new icon themes. It's surprising how much better one feels with a new outfit or face-lift.
Use nicOS-Utility-Suite's Remaster to Remaster. After remastering, you can keep xenial's utilities up-to-date by installing followed by using nicOS-Utility-Suite's Save2SFS. Prefer SFS and portables Web-browers as these are frequently updated and don't have to occupy your SaveFile/Folder. Used incrementally, Save2SFS takes just a couple of minutes. For all intents and purposes you now have a new Xenialpup, fleshed out just the way you like it, usable until web-sites no longer allow access.
Alternative 2: If your computer will support it, frugally add a new Puppy. Estimated time (exclusive of download) 20 minutes. Boot into Xenial for what its best at; boot into the new Puppy for what it's best at. Note, to a large extent portable-apps and to some extent AppImages can be shared.
Edit: See my post 2 down from here