Will future macOS updates affect Time Capsule for Time Machine?

You are on macOS 15.6. Time Machine expects an APFS formatted drive for its backups and the Time Capsule, being a long retired network device, won't let Disk Utility see the drive on its USB port. Even if you plugged that USB drive directly into your Mac Mini, and formatted it APFS, that newer formatting may baffle Time Capsule.

There are two risk factors here.

1. The Time Capsule is no longer sold or supported by Apple and is based on outdated security and technology.
2. Time Machine now expects an APFS formatted drive for backups and may no longer support your configuration of a USB-attached drive to the TC, whether presently or in the future.

I recommend that you do what I do. I have a 2TB Crucial X9 SSD directly connected (USB-C) to my M4 Mac Mini Pro (64GB/1TB). That drive is formatted as APFS and Time Machine backs up to it every hour that the Mac is running.

Secondly, I decommissioned my Apple AirPort Extreme Base Station and replaced it with a Synology RT6600ax router which provides contemporary security and wireless (802.11ax) protocols for better network performance. It also has a USB-C port for a backup network device, or storage. This is an expensive router with an amazing browser-based administration panel, but there are less expensive devices available. I made this decision understanding that eventually, Apple will likely decide to stop including the AirPort Utility with the operating system. That is neither speculation nor fact. It is only useful for managing discontinued Apple network hardware.

Apple generally supports products for 7 years, and the Time Capsule was discontinued in 2018. Time Capsule uses AFP, a storage format that has been deprecated and will no longer be supported for TM backups starting with Tahoe/macOS 26.

You won't see the Time Capsule drive in **, you need to use Airport Utility to format the drive.

Unfortunately, I think your Time Capsule has reached the end of its useful life. Fortunately, replacing it for purposes of a TM backup is easy. The biggest time capsule was 3 TB, you can get a 4 TB HDD for around $100 and a 4 TB SSD (I use a pair of those for some of my TM backups) for $250 or less.

It appears that the issue of support disappearing for the Time capsule is more related to discontinuing support for Apple File Protocol, AFP.

--------

When you place your backups on a Server of any description, including a NAS or another Mac running MacOS 10.13 High Sierra or later, the detailed drive format of the Server drive is a complete 'don't care' as far as Time Machine is concerned. The minimal requirement is that the Server must be capable of creating files on its local drive of Type Mac sparsebundle diskimage.

Inside that Mac sparsebundle diskimage, Time machine creates an expandable disk drive Image to its liking for the backup.

Very true... There are far less expensive routers that are TM enabled and that's a good option. My Asus RT-AC86U is an excellent performer, has two USB external drive ports, and allows TM backups with no muss no fuss... I got it for uner $100. Just about every router now operates from a browser-based administration panel

Grant Bennet-Alder wrote:

It appears that the issue of support disappearing for the Time capsule is more related to discontinuing support for Apple File Protocol, AFP.

👍

The particular file system format used by the network storage device (in this case a TC) is irrelevant.

To be completely accurate it's Apple Filing Protocol. That may appear to be a trivial distinction but filing describes the method by which files are transferred. And it's not to be confused with Apple File System which for some incomprehensible reason instantly became known as APFS.

Confusing? Yes. But APFS is definitely not a TM requirement for network attached storage devices.

How exactly were you informed...?

Connect the HD directly to the Mac mini and do the ** work.

Then see if you can get TM working again with the HD connected to the Mac mini and/or the Router.

for ref: Types of disks you can use with Time Machine on Mac - Apple Support

Any problem with attaching a replacement SSD on the router?

It depends. It is not officially supported by Apple, but if the router runs a version of Samba file sharing that supports SMB3, it might work. It would essentially be a NAS running SMB, so might be close enough. I doubt many routers run a networking protocol that advanced.

The mini is port deficient.

Powered USB hubs usually solve that problem.

Assuming the router supports that, no problems.