Looks like 15.4 is still available, but I would be getting 15.4.1 instead. Im getting update not found when trying to pull any of them, this is going to be a problem on Apples side. Submit Feedback, and if it's direly important open a ticket with Apple.
You can just deploy macOS with VPP as an AppStore app if you need to. You can also package the full OS installer as a .dmg, deploy I with jamf and use some scripting to move the .app where you need it. Deploying the OS as a DMG takes a while to download if you go this route, but it works fine.
Finding available software
Software Update found the following full installers:
* Title: macOS Sequoia, Version: 15.4.1, Size: 15244333KiB, Build: 24E263, Deferred: NO
* Title: macOS Sequoia, Version: 15.4, Size: 15243957KiB, Build: 24E248, Deferred: NO
* Title: macOS Sequoia, Version: 15.3.2, Size: 14890483KiB, Build: 24D81, Deferred: NO
* Title: macOS Sequoia, Version: 15.3.1, Size: 14891477KiB, Build: 24D70, Deferred: NO
* Title: macOS Sequoia, Version: 15.3, Size: 14889290KiB, Build: 24D60, Deferred: NO
* Title: macOS Sequoia, Version: 15.2, Size: 14921025KiB, Build: 24C101, Deferred: NO
@G_Zirrak If you've got a software update deferral configuration profile deployed then it should prevent visibility of a full installer while the deferral is in effect. As an alternative to using software update to download the installer you could the download links published on @ClassicII 's web site: https://mrmacintosh.com/macos-sequoia-full-installer-database-download-directly-from-apple/
@G_Zirrak If you've got a software update deferral configuration profile deployed then it should prevent visibility of a full installer while the deferral is in effect. As an alternative to using software update to download the installer you could the download links published on @ClassicII 's web site: https://mrmacintosh.com/macos-sequoia-full-installer-database-download-directly-from-apple/
I like this app too, "Download Full Installer."
https://github.com/scriptingosx/DownloadFullInstaller?tab=readme-ov-file