Now is a good time to re-evaluate which version of macOS your Macs are running. In the next few weeks, Apple is likely to ship Catalina 10.15.4, which takes it a step closer to its end-point of 10.15.6 in the summer, and whatever 10.16 might bring later in the year. If you’re still running Sierra 10.12.6, your last Security Update was in late September last year, nearly six months ago. Although it’s an unwritten rule that Apple only provides security updates for its current and last two major releases of macOS, you can be fairly certain that all remaining vulnerabilities in Sierra will now be left unpatched.

As a general principle, you should run the most recent release of macOS that you can, without compromising what you do with your Mac. For many, that may not include Catalina, because of its substantial architectural changes including the loss of 32-bit software support.

Some like to point out that each new major, and many minor, version of macOS contains bugs, so the total number of bugs increases with each new release. If that were true, wouldn’t it be better sticking with an older release with fewer bugs? There’s an obvious fallacy here in that this ignores the bugs which are fixed in a new release. In some cases, they may simply be fixes, but in others they occur when Apple completely rewrites or replaces part of macOS. An extreme example is Catalina, which no longer contains all the frameworks and other software to support 32-bit apps. That means that all the bugs in those parts of macOS have gone for good.

It’s also worth remembering that Apple very rarely fixes bugs in older releases of macOS. The only general exception to that is when a bug is also a security vulnerability, and gets fixed in a Security Update. For example, Sierra has a fatal bug in its scheduling system (DAS/CTS) which means that Macs stop making automatic Time Machine backups after a few days of continuous running. Although this was reported to Apple more than six months before the release of High Sierra, it was never fixed in Sierra, only in High Sierra.

What’s far more important is re-assessing the compatibility of all your main apps and other software with latest releases of macOS. Since you last checked, developers may have released 64-bit versions of their products, fixed issues with Mojave perhaps, and even with Catalina. Check through, keeping a record of which still require macOS earlier than 10.15. This should give you a clear picture of which versions of macOS you could upgrade to.

Once you know what’s possible, look carefully at the options. Don’t be swayed by the irrelevant: new privacy controls in Mojave generated a disproportionate number of complaints about all the consent dialogs. In fact, the great majority only occur when you first set certain apps up. Similarly with Catalina, which can at times do some odd things with its privacy behaviour, but those occasions are rare for the majority of users.

Architectural changes are much more important, in particular those that have taken place in APFS. In macOS 10.12 Sierra, APFS is a beta which is essentially useless now. High Sierra 10.13.6 reached at least version 748.51.0, which is fine for SSDs, but isn’t supported on Fusion Drives. Although it will run on hard disks too, I wouldn’t recommend it, and it has some incompatibilities with APFS in later versions of macOS.

It was in Mojave that APFS reached early maturity, with version 945.275.8 running in 10.14.6: that’s a big step in versions since 10.13.6. That will run on most types of storage, although I still don’t think it’s a good choice for hard disks. It’s also compatible with later versions, except in support for Volume Groups and their firmlinks. For those, you’ll need Catalina 10.15.3, which has APFS version 1412.81.1 at the moment. It’s still not my first choice for hard disks, but is fine on SSDs and Fusion Drives.

If your boot disk is a hard disk, then think seriously about replacing it with an SSD if you’re going to upgrade to Mojave or Catalina, which expect to boot from APFS. Although the adventurous might be able to coax Mojave to boot from HFS+ (it has apparently been done), there’s definitely no choice with Catalina, and booting that from a hard disk isn’t a wise choice.

Also worth noting with APFS is that its volumes don’t support AFP file sharing. If you need to share volumes using AFP, you’re best keeping them in HFS+ format, which is of course fully supported throughout 10.12-10.15, as it’s still mandatory for Time Machine backups.

Catalina’s lack of support for old and 32-bit apps isn’t as hard and fast as it may seem. There are solutions for specific apps like Aperture, iPhoto and iTunes using Retroactive, and you can always run 32-bit apps in a VMWare, Parallels or other Virtual Machine, in High Sierra or Mojave perhaps. This works best for apps which don’t make heavy demands on your Mac’s resources which you use less heavily or frequently, but is always worth bearing in mind when making a decision.

Another important factor in upgrading to Catalina is its new boot volume layout, and the implications of that for Time Machine backups. Many of those who have upgraded have lost all their earlier backups, so it’s worth trying to archive those and start a fresh backup set if you can. There are several articles here which consider those problems and potential solutions.

Any robust plan for upgrading also needs an emergency way back: what happens if you upgrade, and only then discover that something vital doesn’t work? Because of its two-volume boot disk structure, it’s very hard to roll back from Catalina, although going back to any version of macOS which boots from HFS+ is going to be difficult when your boot drive has been converted to APFS. Any upgrade from Sierra is going to involve quite a bit of commitment.

At the end of this, you’re likely to arrive at one of three solutions:

• Freeze that Mac on an old version of macOS, 10.12 or earlier. You’re then going to have to plan for its security – either don’t use it online at all, or use a different browser which does have all the latest security features and will be supported.
• Run High Sierra or Mojave with care, keeping up with security updates. Remember though that your Mac doesn’t have the full protection of the current release of macOS.
• Run the current version of Catalina. You’ll still need to install its updates reasonably promptly, as some can address vulnerabilities which are currently being exploited in the wild.

Whatever you decide, I wish you success.